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For pushing multihulls ( or dare I say any displacement yacht or even small barge) you want big blade area, big diameter and fine pitch. Think of a prop moving through butter like a screw. If you are going fast, say 20-30 knots, which is what most outboards are aimed to do, then you need a course pitch to keep the prop gripping at its high speed through the water. But that same high pitch prop will stall the blades at low speed, especially in reverse. A bit like having your sails over sheeted going downwind.

 

The ideal is 'under square' which is less pitch than diameter, but can be hard to find. Much as I really hate to use anything imperial, you should be looking for a 10" x 8", or 9" x 6" etc. Biggest diameter the motor will take, smallest pitch you can find.

 

The ongoing popularity of a the Yamaha 9.9 as a yacht motor is they are one of the few with a low gear box, a big prop for the motors size, and 'barge pushing' prop options. The real difference is reverse and low speeds. But your yacht can't motor at really high speeds anyway; the hull shape won't let it. 

 

PS: Not sure in NZ what a pound is with relation to a boat? Are you a drummer or pile driver?? 

 

Heh heh - Forgive me.  Since I value the input of this board, I do make a respectful effort to convert Imperial to metric with you guys.  However I neglected to this time.  Tennant's specs are 1230 empty to 1767 kilos  (heavy load.)  Current prop is 11 3/4" by 9" pitch.   I don't know if those units apply down there when it comes to props, so that is 29.8cm by 22.9cm pitch.

 

Basically it is first gear....

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Hmmm, so you have roughly 1 m more waterline than I have and 50% more empty boat weight. My guess you will only see a small speed increase from changing prop -- you are limited by waterline length. At 7 knots I guess you notice transverse waves in your wake.

 

/Martin

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 At 7 knots I guess you notice transverse waves in your wake.

 

/Martin

 

It is possible.   I'll know more once I get a few more hours on the boat!   Thanks again for that link.   You've really compiled some interesting information there!

 

Have you had any rot in the core at all?  I found and replaced one wet strip thus far.   No telling how long it had been wet - probably since the boat was built 12 years ago.   But the western red cedar was not rotten at all.

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WRC is not very rot prone to start with.

 

My boat is mostly not WRC since we could not find enough of it on the Swedish market back then (30 years ago).

 

I have had no rot problems worth mentioning or remembering but then I have been pretty quick at patching up dents and whatever to stop water ingress and later on made proper repairs.

 

My main head ache has been UV radiation deteriorating things where I have opted for clear finish. Sooner or later the wood-epoxy bond gives...

 

Clear finish 'indoors' is a pretty good idea though since it facilitates inspection.

 

/Martin

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Heh heh - Forgive me.  Since I value the input of this board, I do make a respectful effort to convert Imperial to metric with you guys.  However I neglected to this time.  Tennant's specs are 1230 empty to 1767 kilos  (heavy load.)  Current prop is 11 3/4" by 9" pitch.   I don't know if those units apply down there when it comes to props, so that is 29.8cm by 22.9cm pitch.

 

Basically it is first gear....

Sounds like your prop is as good as it gets. Not sure how much faster you expect to motor?

 

The limitation is not the prop, but horsepower (lack of) and also a sailing boat (mono or multi) does not motor much faster than 7 knots at this length because of the aft shape of the hull. The stern gets sucked down at speed. A genuine power boat has 'buttock' line relatively flat to the waterline, which help support the weight of the boat and reduces the displacement wave the hull has to climb over to go faster.

 

This is why it is so hard to get a good 'motor sailor' design...

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When ever I'm motoring I always feel like I should chock the motore so that it is negatively raked on the transom and chuck a heap of weight on the bow.  From what you're saying Tim - it seems like that would be a bit pointless?

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Thanks for your comments everybody.   My intention with the original question was to figure out if a higher pitched prop would be proper, given that I have to replace the prop anyway.

 

The general consensus is NO!

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When ever I'm motoring I always feel like I should chock the motore so that it is negatively raked on the transom and chuck a heap of weight on the bow. From what you're saying Tim - it seems like that would be a bit pointless?

I think it's best just to accept that you all have a likely top motoring speed of 6-7 knots, and don't try too hard to go faster! If you want to go fast go multihull sailing!

(See I changed it!)

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