Performance of two boats different shafts..

[Texasmark]

Re: Performance of two boats different shafts..

I agree with steel. You may be mistaking plowing for just john attitude. That boat is going to run flat on the water once on plane and it should plane at 15 mph . You have a very very wide beam and a long length. With the whole boat (except for a little v in the front) in the water, the whole boat will try to steer it'self. Also it probably has a predominant center "keel" running full length which will insure that the boat does the steering for you.
Yes the transom has a pretty steep angle but I don't see triming having much, if any effect on the performance of this boat; the bottom is just too big and too flat.

I think you are expecting something this hull can't give you.

My 2c,

Mark

 

[kaboominator]

Re: Performance of two boats different shafts..

You are both probably absolutely correct. I don't have much experience with jon boats so the science makes sense. Out in it yesterday i could see how a power trim could probably help a bit as it could be lowered for hole shot then raised once on plane to gain a little more speed. And yes it is a lot of boat to be in the water...so we'll deal with the thing and accept it as is..Thank you very much for all your suggestions and advice..

 

[jimmbo]

Re: Performance of two boats different shafts..

I thought 7 degrees from perpendicular to the bottom was the standard for the transom angle.

 

[Texasmark]

Re: Performance of two boats different shafts..

Naw Jimmbo. Mine is 25 degrees and if you go to the web site he gave us and look at it his is at least 20.

And Kaboo thank you for appreciating us trying to help you. 8)

Mark

 

[kaboominator]

Re: Performance of two boats different shafts..

No problem Mark! I appreciate all the great help and advice i receive on here. Who knows i may be answering questions soon too.. I cant keep my hands out of my motor...want to take aprt and put together all the time...addictive

 

[Texasmark]

Re: Performance of two boats different shafts..

You know, you are correct about what goes around comes around.

I have been on here 10 months now and have learned a lot, course I have been associated with boating for 50 years so I came in knowing a few things.

But when you learn something and see someone asking questions about the subject, you can't help but jump in and help them.

I know when they see you jump in, that the folks that taught you are just sitting back with their arms folded just a grinnin about what they did for you.......and rightly so! 8)

Mark

 

[kaboominator]

Re: Performance of two boats different shafts..

update! Well the boat is now perfect. We ended up cutting out 4 inches off the transom and it cruises along and planes perfectly.

 

[Texasmark]

Re: Performance of two boats different shafts..

Back the truck up. If you cut 4" out of the transom, I'm assuming that you lowered the engine on the transom by that amount.

That says that you had a 15" engine on a 20" transom. No wonder it wouldn't run. Once the water left the back of the transom, there wasn't (much of) anything for the prop to engage. Surprised the engine didn't overrev.....course how would you know that without a tach.....other than being extremely noisy.

I reread your initial post and you said two different things about shaft length. Now that it's all said and done, the remark in your last paragraph explains your original problem. You did in fact have a short shaft engine on a long shaft transom.

Other way around will work ok (not stellar, but will work) Your way is a no-no.

Glad you got her fixed.

Mark

 

[gxxdman]

Re: Performance of two boats different shafts..

Start by looking to see where the cavitation plate is in relation to the bottom of the hull. It should be even or slightly above.

 

[gxxdman]

Re: Performance of two boats different shafts..

Hi,

That's fine for a monohull but what about a catamaran? My engine is mounted on a transom between the hulls. The cavitation plate is under the water but of course the water level changes between the hulls when small waves pass between them and the hulls themselves do not respond.

My boat is an old sailing catamaran an 8m Bobcat and I am looking for power not speed but without cavitation. How long should the shaft be? The boat transom height is about 20in/500mm.

Thanks, John (Scotland)

 

[JB]

Re: Performance of two boats different shafts..

Hello, John.

You will get much better response by posting a new thread rather than hijacking an old one.

Good luck.

 

[gxxdman]

Re: Performance of two boats different shafts..

Thanks JB - I already tried that then I noticed that someone was asking a similar question about shaft length conversions so thought I might get something here. I hope someone will be able to help. Best wishes - John (Scotland)

 

[Tail_Gunner]

Re: Performance of two boats different shafts..

Hello, John.

You will get much better response by posting a new thread rather than hijacking an old one.

Good luck.

Id bet there are a few knuckle draggers over in prop forums that could sort this out with startling clarity......just a hint...8)

 

[Texasmark]

Re: Performance of two boats different shafts..

If it were mine, I'd do as I do with a bow mounted trolling motor.......deep enough to keep the prop in the water when it's at it's worst.

Like you said, you aren't out for speed.

Mark