Discussion of HP required to move sailboats....

[Lynx]

I agree on a Venture 242 an 8 to 6 should be fine unless you want higher charging amps then a 15 should be tops.

for info on the thread. I have had to push my 50 hp to 3800 rpms to keep up at 7 mph in wind and seas. However, I have a high freeboard and not much underwater. Bad gas mileage!

[TomRay]

The questions of weight and horsepower have been addressed well, but for outboard powered sailboats a critical concern is shaft length, which I haven't seen discussed. Even with an extending engine bracket, the engine can come up and cavitate when motoring into a chop. There are several situations in which I'd rather have a 6 hp with a 25" shaft than a 9.8 hp with a 20" shaft on a typical small sailboat. More power is no good if the engine is busy cavitating.

[s/v Faith]

Tom,

  That is an excellent point.  There is another thread where that issue (and many others) were addressed as well. 
I posted a link to it in this thread here;

This thread is another good one to read. 

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

  Some discussion of this same topic there too. 

[CharlieJ]

I know this is a losing battle, because the term has solidly ensconced itself in our (as boating people) language,

BUT-

It's highly unlikely that one of our outboards is ever gonna "cavitate". while it is VERY likely they might "ventilate". In fact, the thing up above the prop is known properly as an "anti- ventilation" plate.

Ventilation is when the prop sucks air down alongside the leg, and makes that funny sound we all know and hate.

Cavitation is when the prop is moving through the water fast enough to pull oxygen from the water in the form of tiny bubbles that "explode" on the prop surface, causing tiny craters. High speed racing boat turning many many RPMS have the problem.


Here's the definition of cavitation-"Cavitation is defined as the phenomenon of formation of vapor bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapor pressure."

I know it's a lost cause ;D but I HAD to point this out.

[Captain Smollett]

Used my dinghy as a pusher today to help a dude get his boat where he wanted in the anchorage.  Boat was a Pearson 23 and the ob would not start (he had drug down too close to the channel, and needed to reposition).

Lashed the dink to the quarter and pushed him up.  That's with my 2.5 horse ob on the dink pushing BOTH boats.

Incidentally, this was dead into a 15 knot breeze with gusts topping out around 25 kt.

[TomRay]

I know this is a losing battle, because the term has solidly ensconced itself in our (as boating people) language,

BUT-

It's highly unlikely that one of our outboards is ever gonna "cavitate". while it is VERY likely they might "ventilate"

Push that wet string up that hill, Charlie! ;)

[LooseMoose]

One thing I keep seeing in this thread is everyone is talking HP but there is HP and then again there is HP...

Outboards measure their HP in a different way than people who make diesels or electric drives. The big thing to keep in mind is that outboard HP is very much a product of revolutions on a smalliish prop. and Yes outboards often cavitate on dislacement sailboats unless they are geared low as in the old Yamaha four stroke high thrusts which had a 100 rpm/s less than their normal motors as well as a bigger prop.

George Buehler ( a smart guy) makes the case that a big sand barge can't be pushed by a 200 HP but will move ( albeit slowly) with a 9.9.

My take on HP is from another smart guy, Tom Colvin ( another smart guy) whose HP rule is 1HP per ton for a boat doing coastal cruising and 1/2 HP per ton for a boat doing voyaging. Which seems to work for me but then agin I don't ever sail to a schedule...

[AdriftAtSea]

If the engine is too large and the prop too small or too steeply pitched for a given boat's displacement, there's a good chance it may cavitate rather than ventilate.  A bigger concern is the outboard coming out of the water due to the boat's motion.

[CaptMac]

My boat has a outboard mount on the back and I plan on getting a 25 inch outboard and have been following this thread, my question is what height should I mount the bracket at (25" would give me standard depth) but I want to make it deeper to avoid ventilation, how much deeper than standard would you go?
Thanks

[skylark]

As low as you can.

[AdriftAtSea]

The only caveat being, the lower you mount the engine, the more likely it is to get swamped by a wave pooping the boat.

[Captain Smollett]

Got a 'rescue' request from the Dockmaster today.  A boat from the marina was about 1-1.5 miles away with engine failure.  Winds 15-20 kt with some big gusts and the dude felt overpowered trying to sail.  He anchored and gave a holler to see if someone could come out.

I went on the dinghy.  I lashed on the starboard quarter and pushed him in.  We did not go fast, of course, but we did go.  We were dead into 20 kts with 2-3 ft choppy seas for part of the way, beam-to the rest.

The outboard on my dink is 2.5 HP.

[Tim]

 ;) Just goes to show ya! Good for you, here's a grog

[AdriftAtSea]

Nicely done Capn... what kind of boat was he in???

Got a 'rescue' request from the Dockmaster today.  A boat from the marina was about 1-1.5 miles away with engine failure.  Winds 15-20 kt with some big gusts and the dude felt overpowered trying to sail.  He anchored and gave a holler to see if someone could come out.

I went on the dinghy.  I lashed on the starboard quarter and pushed him in.  We did not go fast, of course, but we did go.  We were dead into 20 kts with 2-3 ft choppy seas for part of the way, beam-to the rest.

The outboard on my dink is 2.5 HP.

[thistlecap]

As for HP, I've routinely run a 31-footer with a 5 HP. Seagull.  On height, my experience has been that the movable weight of a crewman on the boat will make a substantial difference in the engine height.  When I bought Thistle (25 ft. Dufour), it came with an outboard already mounted on a raising adjustable bracket.  The top of the mounting bracket pad was 19" above the waterline, and the engine clamping pad 5" higher, so even in the lowered position, the engine's ventilation plate was just below the water surface.  The very first time I tried to go out, when I went forward to cast off the bowlines, my weight on the bow raised the stern enough that the water pump lost suction.  I heard the engine pitch change as soon as I cleared the shrouds, and ran back to cut the engine, but the damage had already been done.  The waterpump impeller was gone, and the engine was old enough that the lower unit had frozen in place, meaning I had to replace the engine.  I lowered the bracket  7 inches to 12" above the waterline, got an outboard with a longer lower leg, and haven't had a problem since.  The point is you also want to know how much the trim of a small boat changes if you're on the bow tending lines or anchoring.

[okawbow]

It seems like the trend is for more recommended HP than in the past. I recently read a recommendation for about 3 HP for every 1000 pounds displacement. Is that much really needed? I agree that maybe the correct prop size and pitch is more important than HP alone. My 6000# boat has a 33 year old Yanmar YSE8, that puts out 7 HP at full throttle. The only time I've needed full throttle was to get off a grounding. It has been enough HP even against gale force winds and mean chop. If I followed the advice I've seen in print, I would need an 18 HP diesel to move my boat.

[AdriftAtSea]

Maybe because newer sailors are less willing to work with the wind and seas and not as skilled in getting the performance out of their boats as earlier generations of sailors.  There are many times when I'm out on the bay, and sailing along at 3-5 knots, and so many of the other sailboats are motoring to get whereever they're going.

[CharlieJ]

Too true, too very true.

We call them "motor sailors"

Laura HATES and detests cranking that engine, even now since we have the quiet four stroke. I'm almost as bad. We'd much rather do things under sail if at all possible, and 4 knots offshore for our boat ain't bad speed ;D Not for a 17'9" waterline anyway ;)

We almost always come to anchor or get under way using sails alone, unless some real reason exists to not do that. Seldom happens though.

Of course we use the engine when needed, such as transiting the boring GICW run from Galveston Bay to NOLA . But after we get past the Mississippi River, or ANYTIME here when possible, we be sailing.

I feel like those people are truly missing something. I was told once- anybody can sail in 10- 15, takes a REAL sailor to sail in 2-3. :D

[CaptMac]

Thanks for the replies on the motor mount height.

After reading the HP thread I think 6 HP is plenty for my boat, but I also want a 25" shaft but I have two reservations:
1. The only 25" shaft 6 HP is one cylinder (I think two cylinders are safer then one)
2. Most motors at 6 HP have side motor shifters (I have a high transom and would  have to reach way over the back for shifting)

So to overcome these objections I have narrowed my search to either a Yamaha 8 HP (It has a shifter on the tiller and comes in a 25" length) or a Mercury 9.9 (It has the shifter in the twist grip and comes in 25").

I am leaning toward the Mercury because of the shifter in the twist throttle, I think that would be the most convenient, does anybody know of any problems with this arrangement or have any other ideas.
Thanks
Jay

[CharlieJ]

If you are considering the Yamaha 6 HP 4 stroke, may as well opt for the 8 HP instead- it's exactly the same engine, same weight, everything, but has a different carb. If you are gonna tote the weight, might as well have the extra two HP, don't ya think??

Of course our engine lives in a well, so the shifter on the tiller makes it almost like an inboard as far as control is concerned.

All I can say otherwise is - we LOVE our Yamaha. The shifter on the tiller is why we bought it. Did you also know that you can buy longer cables and mount that tiller handle, with throttle and shifter, anywhere you want it, including on the boats tiller?