More Prop Diameter or more HP?

[roland stockham]

I have used the Yamaha 9.9 on a displacement mono with good results. Basically the bigger the prop the more water it shifts so you have a lower speed but larger 'jet' pushing you. The principle effect is that efficiency goes up with prop size at displacement speeds. High speed craft work on different rules. Using an outboard like this is basically the same as using a fixed leg diesel inboard and the same cac'ls apply

[PPLepew]

You have a displacement boat. What's your hull speed??
If it is 10 Knots, you will NOT ever go any faster than that, even with 200HP.
Law of physics can not be bent.

[smj]

Originally Posted by PPLepew

You have a displacement boat. What's your hull speed??
If it is 10 Knots, you will NOT ever go any faster than that, even with 200HP.
Law of physics can not be bent.

Not true.


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[smj]

smj, Yes, I think that your posted picture of a Shuttleworth 35' is the boat which I'd mentioned. Nice design, but for lacking a bridgedeck cabin. But if someone gave her to me I'd muddle through

The beauty of the Tek 35 is it has no bridgedeck cabin yet still has a nice galley and well laid out dinette in one of the hulls! I'm a big fan of the open bridgedeck cats as they are usually an absolute joy to sail.


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[knockabout]

bigger the prop slower the rpm/rule of thumb throttle on full after you are at predicted hull speed the motor wont top out( over rev) sounds like it is still working/
when motor sailing back the throttle off wen the rpm picks up usually end up motor high idle but good hull speed (cast iron apparent)

[Steel sails]

Not sure which motors best but the speeds you are looking for are in my opinion not achievable with the 9.9

I had a 36' PDQ for many years. Went through 3 sets of the Yamaha T9.9 xl shaft. The PDQ is lighter than your boat and realistic speeds in flat water 7 kts cruising both motors 7.5kts WOT
On one motor barely 6 kts cruising.
Rough water significantly less