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Old 08-08-2013, 01:51   #1
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Switching the Prop for an Outboard on a Displacement Boat

Sorry If someone has read my question in a different forum, but I did not get a definitive answer yet.. so thought I might try posting here..

On our 21 feet twin keel sailboat with a displacement of ~3000 lbs and honda 6hp outboard, after scrubbing the hull we are able to reach 5.4kt on level water surface with the wind astern. To be honest I expected to reach hull speed with 6hp in these conditions...

We do wanna make sure we are extracting all the power from our engine to make sure that in adverse conditions (25kt headwind, steep short waves) we could make substantial progress.

Is it reasonable to assume that based on the fact the standard propeller for an outboard is designed to be used inflatables rather than in a displacement boat, by changing the the propeller to a smaller pitch we would be able to extract more power in case we need it?

Current propeller is 9-1/2 x 8-5/8 in.
Honda outboard 6hp BF6B
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Old 08-08-2013, 04:20   #2
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Re: Switching the prop for an outboard on a displacement boat

Struth, 6 hp outboard with 3000 lb boat and getting that speed. Enjoy it. Not that I'd know.
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Old 08-08-2013, 05:09   #3
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Re: Switching the prop for an outboard on a displacement boat

I'm guessing you don't know what RPM it's reaching?

You could try opening it up and listenting to the pitch and then putting it on an inflatable that will plane and listen to the pitch to get an idea if it's reaching peak RPM (if the big boat has a lower pitch, it probably isn't).

If you are reaching peak RPM, there is probably little to gain in top speed with a different prop. If it can't reach peak RPM, a lower pitch prop may help.
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Old 08-08-2013, 05:15   #4
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Re: Switching the prop for an outboard on a displacement boat

I can definitely hear much higher pitch sound when opening throttle in neutral gear .. dunno if it counts

I did order a tachometer as well, so will be able to measure it soon..
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Old 08-08-2013, 05:28   #5
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Re: Switching the prop for an outboard on a displacement boat

Quote:
Originally Posted by parito View Post

Current propeller is 9-1/2 x 8-5/8 in.
Honda outboard 6hp BF6B
Not to be rude, but a Vivacity is a tad on the beamy side (for the length). I guess you were hoping for 6 knots (and maybe 6 1/2 downhill!). IMO 5.4 not bad.

IIRC (it's been a decade or so), on my 21 Foot Corribee I also had a 6hp 4 stroke (IMO 1 or 2hp too unnecessary) had got about your performance. Whilst no great passages under power around 5 knots was the most where boat and engine were happy (i.e. arse not dragging in the water nor engine straining).

But I think in your case a new propeller might make some difference as prop appears to be for a dinghy and not a boat, but the result may well be simply on dropping revs rather than more speed, with the upside that the speed (even if a bit less than top speed) is more sustainable in more adverse conditions...............Unfortunately I have no idea how propeller numbers work!......if you can get a loaner Prop would be ideal!

In any event, likely the biggest restriction on performance in adverse conditions will the boat hobby horsing over waves, prop in water - then prop in air! will mean less speed / progress.
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Old 08-08-2013, 05:52   #6
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Re: Switching the prop for an outboard on a displacement boat

As others have mentioned a tachometer would be very helpful for both diagnosis and maintenance of your outboard. They have a certain RPM range they prefer for efficiency and longevity, which is a much higher percentage of max than a diesel. 6HP should be more than enough for your weight and length. A prop shop can easily advise you further, just take your prop to the shop. If they want to sell you a four blade stainless steel model for $400 think twice.
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Old 08-08-2013, 06:17   #7
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Re: Switching the Prop for an Outboard on a Displacement Boat

Your prop with 8-5/8" pitch would be too much for such a boat, 6" would make it rev higher, analogous to lower gear in a car, and produce around 30% more thrust. A big difference. I doubt the one you have lets the OB reach anywhere near optimum rpm.
Those "sail" outboards usually come with 6" to 6.5" pitch props (my Tohatsu Sail 6hp does).
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Old 08-08-2013, 06:22   #8
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Re: Switching the Prop for an Outboard on a Displacement Boat

The motor is most likely equipped with the stock propeller which is designed for a smaller boat that planes. You definitely will benefit from a lower pitch, high thrust prop that allows your motor to operate efficiently at the peak of it's torque curve and give you the maximum thrust possible.

Most decent outboard repair shops will have a shelf full of various used props and should be willing to loan you a couple if you put down a nominal deposit. You could then do some trial runs to determine which prop works best for your specific needs. There are mathematical ways of determining the proper pitch and maybe you'll run into a mechanic who can help in that respect, but in lieu of that trying a variety of props will get you to the same place.
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Old 08-08-2013, 08:45   #9
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Re: Switching the Prop for an Outboard on a Displacement Boat

Spoke to couple of honda dealers, apparently honda only made this propeller, so replacing to a smaller pitch is not an option.

I was referred to steeldevelopments dot net though - spoke with them, they can lower the pitch by one inch..

Will give it a go I guess.
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Old 08-08-2013, 09:06   #10
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Re: Switching the Prop for an Outboard on a Displacement Boat

Below states 19' 3" LWL, that's about 5.7 knots hull speed. Modern lightweight narrow waterline flat bottomed boats can expect to exceed hull speed by a little when motoring, but I don't think you can expect much more than hull speed on your boat.


http://dialspace.dial.pipex.com/prod...es/v650rep.pdf
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Old 08-08-2013, 09:19   #11
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Re: Switching the Prop for an Outboard on a Displacement Boat

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Originally Posted by cal40john View Post
Below states 19' 3" LWL, that's about 5.7 knots hull speed. Modern lightweight narrow waterline flat bottomed boats can expect to exceed hull speed by a little when motoring, but I don't think you can expect much more than hull speed on your boat.


http://dialspace.dial.pipex.com/prod...es/v650rep.pdf
it's 5.9kt to be exact.

My tests were in ideal conditions (flat water, wind astern) - it's very different when you have 25kt on your nose with short steep waves..

That's why I think it should make a difference
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Old 08-08-2013, 10:36   #12
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Re: Switching the Prop for an Outboard on a Displacement Boat

Quote:
Originally Posted by parito View Post
it's 5.9kt to be exact.

My tests were in ideal conditions (flat water, wind astern) - it's very different when you have 25kt on your nose with short steep waves..

That's why I think it should make a difference
Nothing exact about hull speed, where would you pick on the graph below to say this is where it isn't worth more power anymore. Also hull shape and weight and other factors change the exact shape of the curve.

http://upload.wikimedia.org/wikipedi...-HullSpeed.PNG

Wave-making resistance - Wikipedia, the free encyclopedia


Yes I think that you're right, choosing a flatter prop for pounding into waves should make a difference.
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Old 08-08-2013, 12:06   #13
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Re: Switching the Prop for an Outboard on a Displacement Boat

I replaced the stock prop on my Tohatsu 5 with a high-thrust version, and I love it. It has wider, flatter blades, and it allows me to not only go [very slightly] faster on full throttle, but I can also go slower, which is helpful sometimes for moving around docks and things. Even if Honda didn't make a high-thrust prop, is it possible that someone else did? Or that some other brand's will fit it?

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Old 08-08-2013, 12:41   #14
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Re: Switching the Prop for an Outboard on a Displacement Boat

Quote:
Originally Posted by parito View Post
Sorry If someone has read my question in a different forum, but I did not get a definitive answer yet.. so thought I might try posting here..

On our 21 feet twin keel sailboat with a displacement of ~3000 lbs and honda 6hp outboard, after scrubbing the hull we are able to reach 5.4kt on level water surface with the wind astern. To be honest I expected to reach hull speed with 6hp in these conditions...

We do wanna make sure we are extracting all the power from our engine to make sure that in adverse conditions (25kt headwind, steep short waves) we could make substantial progress.

Is it reasonable to assume that based on the fact the standard propeller for an outboard is designed to be used inflatables rather than in a displacement boat, by changing the the propeller to a smaller pitch we would be able to extract more power in case we need it?

Current propeller is 9-1/2 x 8-5/8 in.
Honda outboard 6hp BF6B
My experience with the power props for sailboat application is that they make a lot of difference.

I'd do what you need to to change the pitch on your propeller. Honda should be selling a flatter pitch prop for that engine.

kind regards,
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Old 08-08-2013, 12:55   #15
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Re: Switching the Prop for an Outboard on a Displacement Boat

Quote:
Originally Posted by parito View Post
Spoke to couple of honda dealers, apparently honda only made this propeller, so replacing to a smaller pitch is not an option.

I was referred to steeldevelopments dot net though - spoke with them, they can lower the pitch by one inch..

Will give it a go I guess.
Not a lot of experience with o/b's but it would surprise me (albeit not completely!) if a manufacturer only made engines that own brand of prop could fit, and even if so for something like a Honda would have thought that the aftermarket would have sorted that .............Or is just that Honda don't make a suitable prop (and therefore the Honda dealer has nothing he is allowed to sell you?).
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