Racing Tunnel hull - need advice on long shaft vs short shaft motor

[brm 60]

Hi there, i have a 12 ft racing tunnel hull. It's a full length tunnel hull. The transom depth is 15" to the bottom of the tunnel, however the depth to the boat bottom is closer to 21". (either side of the tunnel) Can anyone provide advice on whether to buy a long shaft or short shaft outboard? thanks in advance.

 

[roffey]

I can't answer, but your question is confusing me. you sat the transom is 15" but the depth is 21"? are you saying the sponson depth is 21? I would think a jack plate would work best but wait for someone with real advice comes along, lol.

Pictures of the boat would be great.

 

[brm 60]

Hi roffey, thanks for your reply. Sorry for the confusion...pic should help. (attached) the transom to the bottom of the tunnel is 15", to the bottom of the runners is 21. This pic is very similar to my boat (although i don't have an outboard on mine - hence the question of long or short shaft. Thanks again

 

[roffey]

Here is my best advice, you should wait for someone who knows, lol.

I believe you need a short shaft with a jack plate. The lower unit should be just inline with the bottom of the transom, so 15". The jack plate
( https://www.iboats.com/shop/seastar-solutions-manual-jackplate-6-setback.html ) can be adjusted to the exact height you need as I suspect it will need some fine tuning as that boat will ride up on the sponsons. I had a hydroplane when I was a kid with a 10hp so that gives you an idea how much I know about racing boats ... not much. Someone here will give you the proper answer.

That boat looks really fast (100 MPH?) and dangerous. I bet you will really have to know what your doing. Do you plan on racing it?

 

[dwco5051]

That boat looks really fast (100 MPH?) and dangerous. I bet you will really have to know what your doing. Do you plan on racing it?

If your boat is a sport class C running a well tuned light 2 stroke 40 hp you are talking 55 to 60 mph tops. Been away from them for a good number of years relying on my memory for those numbers.

 

[brm 60]

Here is my best advice, you should wait for someone who knows, lol.

I believe you need a short shaft with a jack plate. The lower unit should be just inline with the bottom of the transom, so 15". The jack plate
( https://www.iboats.com/shop/seastar-solutions-manual-jackplate-6-setback.html ) can be adjusted to the exact height you need as I suspect it will need some fine tuning as that boat will ride up on the sponsons. I had a hydroplane when I was a kid with a 10hp so that gives you an idea how much I know about racing boats ... not much. Someone here will give you the proper answer.

That boat looks really fast (100 MPH?) and dangerous. I bet you will really have to know what your doing. Do you plan on racing it?

Hi roffey, appreciate the excellent feedback and advice. I did go with a short shaft 30 Yamaha 2 stroke engine.....weight limit is the determining factor for my tunnel hull (110lbs). The boat in the picture isn't mine, but it's the same tunnel hull design as mine and the stern proportions are similiar to mine. I don't plan to race mine don't worry. ;-) thanks again.

 

[ahicks]

In that pic, not the prop shaft appears to line up on the bottom of the hull (runners)

My vote, looking at that pic, would be long shaft....

 

[brm 60]

If your boat is a sport class C running a well tuned light 2 stroke 40 hp you are talking 55 to 60 mph tops. Been away from them for a good number of years relying on my memory for those numbers.

thanks... I have a 30 hp Yamaha so expecting around 50 mph. Fast enough for me. My Yamaha jetski does close to 65....but i didn't build that...just the tunnel hull one. Appreciate the response.

 

[brm 60]

In that pic, not the prop shaft appears to line up on the bottom of the hull (runners)

My vote, looking at that pic, would be long shaft....

Thanks so much....i was siding that way too.

 

[roffey]

The start of the lower unit ( I think) has to be at the bottom of the hull not the sponson.

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I believe the the lower unit that normally goes on it is a racing unit and is very short. Likely very pricey and requiring upgrades to the motor.
I could be wrong.

 

[QBhoy]

Hi. I think you need to set her up exactly as she is in the photo. It’s likely as she was then, she has a low drag gearbox/nose cone and a cleaver/chopper prop too. Looks like a shorter or modified mid section with exhaust outlet.
But perhaps more than all that…and please forgive me for being the fun police…is it maybe bad of me to say that…if you are asking such a thing…maybe you need to go easy with a wee thing like that. Just to you get the hang of things. That will be a 50 mph machine but feel every bit of twice that…and flighty too. Go easy sir ! All the best.

 

[roffey]

50 MPH on the water is fast or at least it is for me. My buggy goes 55 (on the GPS) and that is fun. The OP, BRM seems to have a handle on it.

I think the short shaft Yamaha is perfect. You can add all the other stuff after you get on the water and take a test rip. Send us pictures of the boat. Did you build it from plans or was it from your head. My hydro was homemade as well.

 

[Scott Danforth]

what is wrong with the TOP and speedmaster lower that is in the pic?

 

[roffey]

what is wrong with the TOP and speedmaster lower that is in the pic?

Not his boat, just a pic of one that is similar.

 

[QBhoy]

what is wrong with the TOP and speedmaster lower that is in the pic?

Nothing. Looks ideal. But don’t think it’s his set up. Think he having a different engine on it