Jackplate And Prop Question

[Reel Poor]

My brother has a 1979 12ft Critchfield hull (looks like an Allison hull knock-off). It has a 8 inch wide pad that runs about 2/3 of the length of the hull, 5 1/2ft beam and a 20 1/2 inch tall transom, weighs probably just under 300lbs. He's going to run an 03 Yamaha 2 stroke 90hp motor with a 20 inch shaft. His question is, how much jackplate offset would you recommend. Also a good guess at prop pitch.

 

[walleyehed]

Re: Jackplate And Prop Question

With no step under there, I'd say it's gonna take 6" .....
I won't even guess on pitch 'cause I don't know the yammy ratio...
Weight distrubution will be important as will the weight of the engine in determining the correct set-back.
Dhadley will be the most informative for this topic

 

[Dhadley]

Re: Jackplate And Prop Question

We also need to know the intended use. Those boats were primarily made for circle racing. Being that short the V4 isn't the best choice. The 3 cylinder Yamaha would be easier to achieve balance with and will still be very fast. A good 2 cylinder 60 will get you faster than most other hulls in that size.

If you're going for strictly top end speed with a stock V4, probably something in the 27" range will be a good place to start. If you're going for 1/4 mile stuff probably something in the 23" range.

With a 20" V4 you'll need a jackplate with as little offset as possible. A Bob;s will give you the most flexability. The wide pad will make it fairly easy to drive.

 

[Ron G]

Re: Jackplate And Prop Question

Thats going to be sweet....keep us posted on this project.

 

[Reel Poor]

Re: Jackplate And Prop Question

Thanks guys.......A little more info here.

The engine is a 3 cyl Yamaha 90hp
Engine weight is 261 lbs
Gear ratio is 13:26
The boat has center steering
He already has a manual 6 inch jackplate but if you were to recommend something else the jackplate is returnable.

The boat will be used mainly for topend straight away speed. Really don't want to do any engine modifications ie, exhaust cut out. He would like to run a thru hub prop.

For initial setup, (low water pickup nose cone installed) would you position the prop shaft inline wit the bottom of the boat ?

Also any suggestions as to brand and style of prop.

 

[Dhadley]

Re: Jackplate And Prop Question

If the plate isn't convertable, I'd exchange it for one that is. You'll be amazed at what the 2.5 to 3 inches will do for balance.

As for a thru hub prop I'd suggest a Turbo. There was a series that they discontinued (Lightning Jr and the Pro Tour) that we might be able to locate. Email me at dhadley50@aol.com -- I may have some suggestions as to where to look.

Most likely you'll end up with the propshaft centered somewhere near the pad. Your balance will determine the exact X dimension along with final prop choice.

Good choice over the V4.

Get a good jacket. A Lifeline preferably, and no -- a ski vest does not count. When you get far enough to test, hook the kill lanyard to the jacket and put tape around the snap. That way, in order for the motor to start the jacket will have to be close enough that it should be on, you won't be tempted to unsnap the lanyard and then forget to re-snap it.

You've got some work to do on the pad. Do it now, not later.

 

[Reel Poor]

Re: Jackplate And Prop Question

Thanks for the infomation Dhadley.

I made a mistake on the jackplate, it is hydraulic.

As for the work on the pad that you mentioned. This boat is going to go thru a total recondition which will take care of any imperfections on the pad and everywhere else also. If you were referring to any other kind of work would (shaping or configureation) you please elaborate. The plan is to get the edges sharp and straight and the pad surface as flat as possible.

Thanks again

 

[Dhadley]

Re: Jackplate And Prop Question

Great plan, you're on top of it!

 

[Ron G]

Re: Jackplate And Prop Question

Are ya'll going to be racing or playing or both.that thing should be awsome.

 

[Reel Poor]

Re: Jackplate And Prop Question

Dhadley if you don't mind I'm going to save you e-mail address and contact you as this project starts comming to an end.

Ron, it's not really being built to race.....weeeell, unless ya ont to.

Primarily being built as a river/lake sport rocket. Should be a blast. He's hoping to see mid 70's out of the box.

 

[Ron G]

Re: Jackplate And Prop Question

Cool,when ya get it going i want to some pics of it running.

 

[Dhadley]

Re: Jackplate And Prop Question

It should be a great project. Keep us posted! Here or via email is fine.

 

[Reel Poor]

Re: Jackplate And Prop Question

Thanks again.

BTW thought you might like to see some pics of his last project. It's a 1949 Neal, completely restored and slightly modified. It's powered by a Yamato 302, and runs out in the low to mid 60mph zone.

 

[walleyehed]

Re: Jackplate And Prop Question

What a piece of work!
I hope the wood on my Hydro comes out looking that good!