Prop. height

Hightower · 02 April 2004, 20:06

Put quite simply. How do you know what position your prop should be in relation to the hull? Trial or error!

Everytime I walk pass my Directors, Honda 4stroke race boat (about 20 times a day), I can't help notice that the prop and cavitation plate look way too high. The prop is above the bottom of the transom, this can't be right! Can it?

Andy

Alex Brown · 02 April 2004, 20:14

Sounds like a surface piercing prop setup.

I know very little, but I'm sure there are others on this forum who can explain, either that, or look on google or something:-

http://www.google.com/search?q=surfa...UTF-8&oe=UTF-8

-Alex

Richard B · 02 April 2004, 20:26

Quote:

Originally posted by Hightower
Honda 4stroke race boat (about 20 times a day), I can't help notice that the prop and cavitation plate look way too high. The prop is above the bottom of the transom, this can't be right! Can it?

Andy

Is it attached to a jacking plate? If so, it can raise/lower the engine. I don't know if these are used on Honda 4st or not.

Pete7 · 02 April 2004, 21:34

Quote:

Trial or error!

Yup. Start with the anti cavitation plate about 1 inch above a line that would extend out from the base of the rib hull. If this works great, and if you want to try and go faster then raise it a hole.
If the prop spins and looses grip then its to high. S/S props allow you to run a higher prop and in theory faster but the prop will ventilate more as you go higher particuarly when making sharp turns or zooming across the waves. If youwant to go really fast buy one of those speed boat thingys.

Long shaft engines are built for a 20" transom. Opsrey knowing you would start at 1" up decided to build 21" transoms on there ribs to make life easy for owners.

Pete

Hightower · 02 April 2004, 21:39

No, it's not on a jacking plate.

The race series this year are running 150hp units that seem to be higher than the 130hp ones from last year, but that said the 130hp outboard sat quite high as well.

I guess what I really wanted to know was what height is the correct one?

Surface piercing props.......are they used when you can't get the Engine any lower, or do they add any special significance to the performance of a set up?

Pete7 · 02 April 2004, 22:05

Think you need someone like dirk to answer that one, but they are available for your Yamaha.

Pete

Hightower · 02 April 2004, 22:22

Pete,

you know that we talked about the fact my prop seems to be oversize (pending mechanical/electrical issues) the other week. Someone I was speaking to the other day said that if the engine was raised, that would increase the revs. Do you think that's right? Perhaps he was thinking of surface props!

Need to concentrate on getting Kohaku into the water first though.

Andy

Pete7 · 02 April 2004, 22:30

I think you would be better off with a 2" less pitch prop to get the revs up. this will increase the acceleration stop overloading the engine. If you raise the engine to far the its a pain to drive as it will keep loosing grip and you have to stop and accelerate again. Been there done that then put the engine back to were it should have been in the first place.

Pete

Hightower · 02 April 2004, 22:39

Really just a case of getting my ass in gear, having some nice weather and get all that work done. My Easter deadline is not looking too good at the mo. Will keep you informed.

Andy

Cookee · 03 April 2004, 06:47

Quote:

Originally posted by Hightower
Pete,

you know that we talked about the fact my prop seems to be oversize (pending mechanical/electrical issues) the other week. Someone I was speaking to the other day said that if the engine was raised, that would increase the revs. Do you think that's right? Perhaps he was thinking of surface props!

Need to concentrate on getting Kohaku into the water first though.

Andy

The Honda props are definitely not surface piercing props although most props can work quite high on the transom.

Be very very carefull when raising engines high - you must have sufficient water pressure for the engine to cool - Race engines have the water pickup mounted on the nose of the gearbox - leisure engines get thier water from the side of the gearbox several inches higher!

Raising the engine will give you more revs if it is under revving before, and will probably make you go faster as well. It will usually give more slip on acceleration so won't be as good for skiing for instance!

We run lifters on the raceboat because of the benefits, but we will only fit them on the leisure boat by request as they are very expensive!