Optimal outboard pitch

[Adamhagan]

I have a 1999 Yamaha 6hp 2-stroke pushing my 1981 CD25. In dead calm waters of the ICW with no tide I can make 3.8 knots COG. Factor in unfavorable tide, like my trip into the Norfolk harbor with outgoing tide from the surge of Irene being passed by Navy warships and a sub while I was struggling to cover 1.5knots. A bit more power would have been nice.

I am otherwise happy with the engine, recently had a major service, charges my batteries, and one pull start.

Do I need a larger outboard or what is the optimal pitch I should have?

[sfreihofer]

Hi Adam;

I have an 8hp Yamaha for my CD25. I haven't used it because it came with the boat and I haven't launched yet because I'm rebuilding. I expect to launch in a month or two... when these pesky hurricanes go away.

The engine came with a speed prop. When I rebuilt the engine, I retired that as a spare and switched to a power prop.

The speed prop has swept (curved) blades, and you can tell just by looking at it. The power prop is a slightly larger diameter, slightly less pitch, larger wetted surface (bigger blades) which are not swept.

Ask your Yamaha dealer what prop options there are for your engine. You may want to switch to a power prop, since you certainly aren't going to get much speed in a Cape Dory!

I can't offer a comparison because I've never used the speed prop or the power prop. For that matter, I've never even used the Cape Dory. Yet.

Stan Freihofer
1981 CD 25 #794: S/V Pearl
Ft. Lauderdale, FL
www.reefroof.com

[Klem]

Props can have improper pitches and areas (I am ignoring blade design). A combination of pitch and slip (function of area) will give you a relationship between boat speed and rpm for a given set of conditions.

The most common problem is an improperly pitched prop. If you have too much pitch, it puts too large of a load on the engine and the engine cannot reach its rated rpm. I assume that you don't have a tach which means you either need to hook one up (spark plug ones are very easy) or have a good ear. An underpitched prop won't put enough load on the engine. A properly pitched prop will allow you to reach full rpm but at very close to full load.

Your motoring speed sounds low to me given the hp and the boat size. I think that you are on the right track to look into the prop.

[tartansailor]

You want a 6" pitch, 5" if you can have one made up.
Why?
Because that will allow your 6 hp 2 stroke to reach optimum (5,000) rpm
My guess is that you presently have something around an 8" pitch.

Dick

[Duncan]

As in previous comments, I think 6 hp ought to be good for your boat, so it must be either the prop, or a power problem with your engine.

Before my CD 27 (with a 7 hp diesel), I've had two-stroke outboards on boats that had about the same waterlines.

On one, with an 18' waterline and 10 hp, the boat would hit hull speed at 1/2 throttle. On the other, with more waterline (20') and less power (8 hp), I would be running at about 3/4 throttle to hit hull speed.

I've also sailed on the latter boat, with the new owner, using a 4 hp, and it was struggling.

So, without getting too fancy about it, I'd say 10 hp is too much and 4 isn't enough. I'd be inclined to stick with your 6, since 2-strokes like to run high in their rev range.

But something is keeping your 6 hp from giving you the speed it 'should", so I'd be inclined to check the prop.

[Adamhagan]

I too favor the pitch idea; first because it is the cheaper solution and second it sounds like there is no load on the engine at full throttle.

Is is an long shaft outboard and I have the original receipt but it did not specify pitch. I will hopefully make it out to the boat next weekend before I finish my journey down the ICW to its new slip.

I will post on here my finding and conclusions.

As always thanks to all the wisdom I have been given by the Cape Dory club, I already had to take out a 2nd on my house for Jim & Andy's help!

[sfreihofer]

Yamaha only offered 2 props for my engine, if I remember correctly.... a speed prop and a power prop. Assuming that Yamaha has done their homework, they have matched their props to their engines and you shouldn't have to worry about it.

To say it again, I would check with a dealer and see what props they offer for your engine, and pick the one best suited to a displacement hull... a power prop, not a speed prop.

Stan

[Adamhagan]

I made it to the boat today in preparation for the last leg of delivery. The prop was encased in barnacles probably adding to the loss in efficiency I've seen. This was surprising since I have a receipt from the PO for a local shop that did a overhaul in May, either they did not clean it up or those suckers grow fast in the Chesapeake.



Either way, I could not identify any markings on this worn prop and as Jim Davis stated they only make two props currently:

"A six should be more than adequate for a 25. I would try a prop with less pitch, Yamaha used to make one (3x9x5) but they seem to have dropped it in favor of larger diameter with 7" pitch. The Dual Thrust seems to be a change in the exhaust to get it clearer from the prop. The 3x9x7 may work, can't hurt."

I will get the new prop and will keep the outboard out of the water when not in use.

Any thoughts, could it have been the growth alone?[/img]

[Adamhagan]

http://im1.shutterfly.com/media/47a1d73 ... 0/ry%3D480

[Jim Davis]

Yes this has been a good year for them here on the Chesapeake. Good for them, bad for boat owners. The little suckers have proliferated faster than rabbits. The fouling you show in the picture would definitely hurt your speed. There was also probably a fair amount of grass/slime, which would also hurt. From my experience the best solution is to yank the engine when you aren't on the boat for a week or so.

Other workable things would be anti fouling paints (fair), zinc paint (cold galvanizing fair) and water pump grease (poor, but easier to clean).

On the two props Yamaha offers one is 8 inch diameter the other 9 inch. The exhaust hub seems to be the only other difference, but I haven't seen enough of these to be able to tell which you have. The picture, however should be enough for a dealer to tell.

[Terry]

This discussion is very interesting. I've been thinking for some time of replacing my 1977 Evinrude 6hp that is on Cassandra (CD25).

I've chosen the Tohatsu 6hp SailPro, which has a 8.375" Dia. X 6.0" Pitch prop. It also has a 25" shaft and installed charging circuit.

I assumed this "high-thrust" prop would be adequate, and better than the standard prop, which is 7.7" Dia. X 8.0" Pitch.

From the postings on this thread, I think it should work pretty well. 'Course I still have to either drop the mounting or raise the hatch to fit the taller motor in the well.

I really enjoy the discussions, and applaud y'all.

Lord Bless, Keep, Shine. . . Terry

[Andy Denmark]

Adam,

That picture shows considerable fouling. Before you purchase another one, save the $$$, clean this one thoroughly, and give it another try. I think you'll be pleasantly surprised.

Barnacles grow rapidly here and in the Cheapeake. I'd make removing the engine a part of your shutdown procedure when leaving the boat for more than a couple of days.

I'll be here when you arrive.

Andy

[Adamhagan]

Andy: That was my hunch, instantly upon pulling the engine out I had visions of the turbulent water those critters must have been churning up. I have a long weekend to head to your place, so let the science experiment begin ( before my current gig I was a science researcher, so we might as well).

I have read all the horror stories about outboards left in the water and while this one is still running good I will keep it topside while not in use.

Terry: I was on another CD25 with a 6hp Tohatsu and we didn't have a GPS or hull speed but it felt like it pushed it through great, which is why I was so surprised by the lack of power on mine. Well see what happens now that I've knocked all the freeloaders off.

Again, I am thankful for the greatness of this forum and the kind people who comprise it.