Discussion of HP required to move sailboats....

[matt195583]

mrb yes my boat is an Aussie made Catalina 22 and my 10 is pull start with no charging , I was considering putting a rectifier on it to charge but apparently at the RPM i run at it will do next to nothing. my electrics are fairly basic with fish finder, tiller pilot, fluro cabin lights and LED nav lights, 27 meg, bilge pump and a 35 liter Engel fridge. I want to fit a VHF stereo and a GPS also . I have a 10 watt solar panel that keeps 2x100 amp hour battery's well topped up so I'm not sure if i need charging capability at all.

the weight is a concern but what is more of an issue is the excess power , with my brother at the tiller the other day. He kept wanting to screw throttle on despite me telling him that hull speed was it  >:( . and i know for a fact that of too much throttle is applied the motor mount will actually flex the transom . I am looking at reinforcing the mount but its harder with the inner skin in the way .

So that flexing issue is probably the main reason why i am considering the HP change also the risk of oiling up from low rev use . extra electrical power would be a bonus . 

[mrb]

Matt

In those circumstances I think you are one the right path, trade that monster off.

My boat is a com-pac 16 which displaces around 1100 pounds.  I have a tohatsu 3.5 hp motor which pushes it up to near hull speed at a high idle setting.

A 5 should do you just fine.  Funny how some of those builders are right on sometimes.

I also know how it is when trying to explain hull speed and motor rpm to some.

Good sailing down there
mrb

[Publius]

i have a 26 ft swing keel, how much HP should i be looking for in a outboard motor?

[Tingira]

Hi there, I have been doing a lot of research in this area ( most of it on this site and a few others ).  Not sure what / where you are looking to use it, but I would say your probably fine with a 6 hp.

[matt195583]

I have 10 HP on a 22ft swing keel and it is about 6 too many .  :)

[wlshor]

I have a 26' fix keel shoal and I use a Nissan 4 stroke 9.9 which does a great job. ;D

[Lynx]

It mostly depends on your experience and your cruising grounds.

No current and little head winds. a 6 HP will do.

Lots of experience - a 6 hp will do.

Strong current,
Strong head wind and seas,
Not much experience, Get the bigger motor. I know of a man who had a 25 HP on a MacGregor 25. He was in the Bahamas Nov 07 to Mar 08.

The bigger motor will handle a lot of operator errors. A smaller motor can work well. a lot of discussion on this forum about this. Make sure that you get a high thrust prop (low pitch).

[s/v Faith]

It mostly depends on your experience and your cruising grounds.

No current and little head winds. a 6 HP will do.

Lots of experience - a 6 hp will do.

Strong current,
Strong head wind and seas,
Not much experience, Get the bigger motor. I know of a man who had a 25 HP on a MacGregor 25. He was in the Bahamas Nov 07 to Mar 08.

The bigger motor will handle a lot of operator errors. A smaller motor can work well. a lot of discussion on this forum about this. Make sure that you get a high thrust prop (low pitch).

James, I don't know how to put this any more tactfully, but you are wrong.  I say this not to argue, but for the sake of those who may not know.

  Other then a hull like yours, designed to plane in one mode and sail in another, 25 hp is not only not helpful on a 25' sailboat, but dangerous.

  The boat you posted about earlier shows exactly why this is such a problem.  A displacement hull will not respond well to attempts to cause it to exceed hull speed.  There are times you can do it, when the conditions are right.... but the application of horsepower beyond the fixed mathematical formula causes more problems then it solves.  While we have largely discussed issues of weight and fuel use, there is a valid safety concern as well.

  Too much hp will cause the bow to 'dig in'  This resistance eats up the hp, so it does not translate into anything productive.  In calm water, it only makes the boat 'squat' maybe lowers the stern... in a seaway, it can be disastrous.  Plowing into waves with too much hp applied is going to cause you to have a very rough ride... as born out by  example you cite.

  Please, anyone considering this do yourself a favor and re-read this thread from the beginning.  IF you have a Mac, or a non-displacement hull (multi) put whatever you want on the stern*... if you have a regular displacement mono hull less really is more*.


* Within the rated HP range
 

 

 

[s/v Faith]

This thread is another good one to read. 

Outboard motors; Cruising, dingy, tips, maint, & reviews

  Some discussion of this same topic there too. 

[AdriftAtSea]

No, it actually depends mostly on what boat you have.  I'd have to agree with s/v Faith on this point.  Overpowering a smaller sailboat with a displacement hull is both unsafe and a waste of money/fuel.

It mostly depends on your experience and your cruising grounds.

No current and little head winds. a 6 HP will do.

Lots of experience - a 6 hp will do.

Strong current,
Strong head wind and seas,
Not much experience, Get the bigger motor. I know of a man who had a 25 HP on a MacGregor 25. He was in the Bahamas Nov 07 to Mar 08.

The bigger motor will handle a lot of operator errors. A smaller motor can work well. a lot of discussion on this forum about this. Make sure that you get a high thrust prop (low pitch).

[Lynx]

I do agree with you Faith. A 25 hp was a bit over the limit. I think the most he should have gone with was a 15 hp to push through wind and waves. If he had a 6 hp we would have not made it to Bimini as the last 12 miles was dead into a 20 knot wind and 4+ foot seas.  It was rare that I needed more than 1/4 the HP of my 50 hp but in those times when I was not making headway and I wanted/needed to it was good to have. There is a major thread on this. My boat has a high freeboard and not much underwater unlike someothers and the extra hp sure made a safer  1 year+ voyage of over 5500 miles.

My point to make is to go with the biggest HP motor that your hull can take to help you get through those situtations you put yourself into safely. I do recommend that you find others of your make of boat and see what the biggest motor used and what happens with smaller motors going into wind and waves.  I must admit that when I remotor I will go with a smaller motor or a much bigger. The 50 hp is overkill at displacement speeds.

[Captain Smollett]

If he had a 6 hp we would have not made it to Bimini as the last 12 miles was dead into a 20 knot wind and 4+ foot seas. 

I know we've been over and over this, so I'll try it from a different angle.

What basis do you have that he could NOT have motored into 20 kt winds and 4 ft seas with a 6 HP on a one ton boat?

Because I HAVE motored into 25 kt winds and 4 ft seas on my 4.5 ton 30 footer... with a 8 HP outboard not even running at full throttle.

I guess what I am saying is that on consideration of the experiences I have had, I reject your premise that it is an underpowered outboard that causes problems in 20-25 kt winds aboard some boats.  Mine will get me there.

Still, though, given searoom, I'd rather sail than motor anyway.

[rtbates]

While I agree that having ample engine power is a very nice thing to have, relying on that power to get you out of trouble is a recipe for disaster. Engines do quit.

The old wisdom was 2hp per TON. Now days I believe it's 3hp per ton.
I sail a 6000lb Cape Dory with a 6.5hp Yanmar diesel. I've never NEEDED more. At times I would have liked more...

One day last winter we were sailing in 40+, Coming back in I had to increase the Yanmar to 2k rpm in order to get the bow to tack through the wind. Going dead to windward was not an option even if I had 100hp. The ride was just too wet and rough. By motoring a few degress off dead to windward all was swell.

[Captain Smollett]

By motoring a few degress off dead to windward all was swell.

Excellent.  Grog to you, sir!

Therein lies the problem.  I think too many people refuse to work WITH the boat and the conditions and try to force a bad line.  That can be as simple as trying to sail to a schedule or perhaps running off when heaving-to might be better.  Or, not working with the tides.  Or, motoring dead into the wind when a few degrees off is the better, more comfortable and safer course.

[Tim]

I think I have come up with a stock answer for this discussion,


"for Posedin's sake man, it's a SAILBOAT"

[CapnK]

I have 6hp 4 stroke outboard with a standard prop (not a high thrust) mounted in an outboard well for a full-keeled boat w/displacement of 5120#'s (manufacturers number). That 6 hp engine has done me fine in lots of weather and strong winds, except for once - in the winds out in front of an approaching hurricane. It was blowing sustained over 25, gusting to 35 or better, and I was running both straight upwind and upriver against ~3 knots of current+tide.

In that situation, the boat would not move dead straight upwind running forward (in reverse, she would - but then the engine was towing the boat, not pushing it, and the going was very slow...). She would fall off the wind+current to either side about 45 degrees, and only then I could make progress. "Tacking" under motor power, you could say. I was able to make 2-3 mph over ground upriver despite not being able to do it in a straight line. So IMO, 6hp should be plenty sufficient, especially if you'll get a high-thrust prop.

-----------------

Regarding X amount of horsepower and displacement hulls and whatnot: Look at a photo of a boat with a displacement hull (or a planing hull, going at displacement speeds), and you will see 2 distinct waves along the hull; one at the front of the boat created by the hull pushing water up and out of the way, and the second wave at the after part of the hull where the bow-wave water has 'fallen down' and rebounded back up.

A displacement hull is forever stuck between those two waves.

A planing hull has enough power and speed and is designed specifically to lever itself up and over the first wave and begin 'surfing' across the top of the water out in front of the very wave the hull itself is making. While having the power available to do this is important, having a hull shaped to optimize the process is equally, if not more important.

For most sailboats, in particular those not designed to ever leave the area between the hull waves, the amount of horsepower needed to drive the hull at basically a mathematically proven/predicted speed is pretty small. Going over that amount of horsepower becomes somewhat of a self-defeating strategery - you carry more weight (in fuel and parts) to go faster, but it takes more power to move that more weight.

Given calm water and winds, if all it takes is 6hp to move a given hull/boat at its maximum speed, then a 25hp motor is not really going to be able to make the boat go much faster at all than the 6hp engine. It's not because there isn't more power available, it is because of all the other factors - things like prop type and size, hull shape, displacement and weight distribution, etc, so on and so forth.

A sailboat (well, most of them :) ) is designed to perform best while staying between the bow and stern waves. Because *this is how it is designed*, you can't just slap a much bigger motor onto the same sailboat in order to make it go much faster. It simply doesn't work that way.

I could put an engine into my boat that would make it go faster than "hull speed" - a military jet fighter engine has more than enough power to make my boat go quite fast!  ;D BUT - I would also have to make sure to do all the other stuff a naval architect does in order to make it work: put the engine in the right place, make sure it is geared properly so that the propeller - the carefully chosen propeller - can drive the mass of the boat optimally, make sure that the hull and fittings on the boat could stand the stresses of that much power *and* the stresses that would change with the boat moving through the water at much different speeds than it was originally designed for. I would have to change *the whole system*, not just one part of it, in order to meet the goal of speed..

Most of the boats like mine originally came from the manufacturer with a 30 hp inboard gas engine. While that is 5 times the hp of my outboard, I can guarantee you that one of those won't go even .5 knot faster than my boat under power in calm wind and waves, despite having 5X the horsepower.

What might be different would be in those pre-hurricane conditions; There, that engine would probably be able to go straight upwind - but still not faster than hull speed. It would be able to do it because of the design of the drive train was likely optimized to produce the thrust needed in those kinds of conditions.

Whereas my 'outboard in the well' is being used for a much different purpose than what it was designed for. I could improve it's capabilities by changing the prop to something more suited to the use, but even so it will not be optimal, because the engine itself was not designed for the task it is doing.

The only outboards I can think of offhand that might be would be the British Seagull line, or perhaps the Johnson Sailmaster series (which I am not very familiar with). I do know that the Seagulls were specifically designed to produce the high thrust needed for pushing displacement hulls, with engines and props suited to the specific task. I would love to see and hear a Seagull run, because I have heard they are noisy and smelly. I do know they are also dirt simple, reliable, and designed to do just what I need from an engine, so they are still not off my list of possible auxiliary engines.

So think of it as a 'auxiliary propulsion system'***, and consider not just the question of 'how many horsepower', but also the things needed to properly apply whatever horsepower you *do* have - prop, engine placement, the weight involved and how it is distributed and what effect that will have on how the hull/boat performs. All these things are a part of the question.

Well, that turned into a long ramble, for no particular reason! ;D But maybe it'll help someone someday if they stumble across this thread...

-----
*** (note the word "auxiliary", as in 'functioning in a subsidiary capacity' - these are first and foremost 'sailboats', after all... :) )

[CharlieJ]

Good  post.

In regards the Seagull. Had one. Noisy, smelly and drips oil The early ones had a 25-1 fuel to oil ratio and much a pretty good sheen on the water. The later ones went to 50-1 which is SOME better. Another point is vibration. I had a Silver Century 5 HP on a San Juan 21. The engine would vibrate the screws out of the fore hatch hinges. Every once in a while I'd see one dancing around up on the fore deck >:( Not fun. In addition you COULD NOT hold a conversation in the cockpit while the engine was running.

We'll keep our 8 hp 4 stroke on our 7500 ( loaded disp) boat, thank you very much. The 6 HP version would have been totally sufficient, BUT- the 6 HP Yamaha is exactly the same engine as the 8 HP, weight wise- they just change the carb, so we figured if we had to carry the weigth, we might as well have the HP to go with it. Beside- it was the ONLY Yamaha available in the entire southern half of Louisiana when we bought it. We were broken down with a shot 2 stroke and HAD to get an engine.

The only time I run it past the "start" setting is once in a while to blow it out for a few minutes, and we accept the squatted stern during those times as something to live with. Oh, by the way- from a medium idle, to full WOT gives us .1 more speed. ONE TENTH of a knot.

[Marc]

CharlieJ, you are like my nentor,  I think I will go with and * hp but how does one determine how big of heavy you can put on a boat?  I have no plates on there telling me anything.  I'm just hoping that an 85lb 8 hp will be oka. the PO had a 9.9 merc two stroike if that means anythin.  And as far as I want to travel I need fuel economy and torque not speed when I need it.  I'm still open to suggestions.  Marc

[Lynx]

Good discussion and good points. It is good to see that your boats are performing well in winds over 20 knots under power. Most sailboats with inboards are not fairing as well. People are saying they cannot go into 20 knots of wind under power if they had to. I have just pushed it past the limit a few to many times and had to use the extra hp to get me through that bridge or squall or into or out of a harbor or make that port in time. Voyaging is the extreme sport of sailing. No need to take the extra chance if you do not need to, but then again, why spend the extra money if you do not have to.

A very interesting thread on how much hp is excess.

[CharlieJ]

 ;D

Lynx-

I remember reading some place that the Hiscock's had an 8 hp gas inboard in Wanderer III. He was commenting that going into some of the passes in the south sea islands he wished he had a larger engine- he was talking about putting a new one in- a 10 HP ;D ;D

I had a 20 HP diesel in my 35 foot trimaran. LOTS of windage. I can't recall a single time when we ran that at more than about 2 k RPMs, and never had any troubles in 2 + years of cruising the east coast, Annapolis to Dry Tortugas, then around the gulf to Galveston. I can recall times when it wasn't fun, and times we weren't moving very fast, but it was a great HP for that boat. In fact we could have done as well with a 15.  A buddy of mine ran a Piver AA 36 tri for 4 years in the Bahamas, with a 10 hp Honda in a well in the cockpit.

I CAN recall times when we both just kept the anchors stuck for a few days because it was totally senseless to try to move. :D

Marc- I'm not familiar with the Venture 242, but I think it's a fairly light boat. You will pay a penalty for having the weight on the stern, but some of that will be offset by weight up front. I know the gurus talk about keeping the boat light, and balancing with most weight amidships. Fine, until you take off cruising. Then all bets are off. I've NEVER seen a loaded up cruising boat that was floating on her lines. Usually they are at LEAST 2 inches down ;D I mean, it takes a lot of *C*R*A*P to keep people when away form an easy home port .

So if you are just weekend sailing, that 8 HP) 4 is probably gonna be to much most times. But if you will eventually take off for a while, you'll never notice that weight..

Of course, one advantage will be you'll not need to carry as much fuel for a 4 stroke than you would with a 2 stroke. Like you can go twice ( or more) As far on the same fuel load.

We had looked at a four HP 4 stroke. Two reasons we chose the 8- 3 actually.

Smaller engines were all single cylinder- the 8 hp ( and the 6 hp Yamaha ) are the smallest 2 cylinders they make in 4 stroke, and the 6 is the same engine- same weight.

Secondly, we had limped into ports on the ICW running on one cylinder on two occasions. You ARE NOT GONNA sail the ICW in Eastern Texas and Western Louisiana, unless you can afford to wait for weeks for the right winds. So a reliable engine is a must there.

the third reason is that we were sitting in Intercoastal City with a crashed engine ( the second one, both 2 strokes, on that trip), needed SOMETHING to get home on and that 8 HP Yamaha was the ONLY engine of it's type available in the entire southern half of Louisiana . We really didn't want to spend the bucks, but we gritted our teeth and handed the man the credit card ( that's the only time we use credit cards by the way- urgent stuff)

By the way- if I sound sold on Yamaha's you are correct. For one thing, on our boat with the engine in a well, the shifter and all other controls being on the outboard tiller is great- handling is about like using an inboard controls. And they have a very good reputation for reliability. If it was on a mount on the stern, I think it would STILL be easier to use

Oh, and Marc- if you have not already done so, I'd recommend you go back to the first page of this thread and read the entire thing. VERY interesting and informative