Mounting height for jet drive?

[M700]

I purchased a 1652 G3 MV jon boat with a Yamaha F60/40 4 stroke on it with a 25 inch shaft. The transom is a 20 inch. The factory rigged it. The motor sits up about 8 inches on a set of CMC vertical extensions. The Yamaha specs call for 5 inches of vertical raise to mount the jet motor. Right now with the motor trimmed in all the way, about half of the jet foot sits below the boat, I put a level on the bottom of the boat to see this. Dealer says it is fine but I would like to know why it sits higher than factory specs. Is there a rule of thumb on how far above or below the bottom of the boat to set the jet foot? The motor weighs 264 lbs, so in this 16' foot boat it will squat some. Any one with mounting jet outboard experience like to comment? I would like some input from someone with hands on experience. Sorry for the long post. Thanks, John

 

[dick]

Re: Mounting height for jet drive?

This isn't the best picture but is the front of the intake level or just below the bottom of the transom ? Does your boat have a tunnel built into it ?

<br />If the engine dosen't ventilate in corners and theres not a huge spray behind the boat the height is probbly just fine.

 

[Bear]

Re: Mounting height for jet drive?

No experience, just links. I think this one answer some of your questions:<br /> http://sites.state.pa.us/Fish/sepoct99/jetboard.htm <br /><br />From here:<br /> http://www.scottandjenn.com/outboardjet/ <br /><br />I Hope it helps.

 

[Forktail]

Re: Mounting height for jet drive?

I've run a few jets and can tell you that mounting height is extremely critical. And every boat/jet combination will act differently.<br /><br />The best advice would be to mount it on a transom jack, giving you the ability to move it up and down according to load and water conditions. One disadvantage with this is even more added weight than you already have, and that weight being even futher back on the boat. One advantage to the transom jack is that you usually get more power out of the jet because of the water flow there.<br /><br />If only half of your foot is below the boat bottom, I would guess it's too high. I remember the leading edge of the intake grate being level with the bottom. Sometimes even a 1/4" can make a big difference. You'll just have to try it.<br /><br />Remember, the bottom of the boat and how the water flows off of it, is just as important with regard to performance. Expect cavitation from time to time, but in general the outboard should maintain it's "grip" even when turning.

 

[M700]

Re: Mounting height for jet drive?

I don't have a tunnel hull, just a normal transom, with a 6 degree vee to it. After looking at the diagram posted and the other advise, it looks like I am about 3/4"+ too high. The beauty of it is I have a lot of adjustment in the mounting holes on the motor since it is mounted in the highest position and I have plenty of downward adjustment. I think I will try lowering it one hole and that should get me in better shape. Problem is, is, everything is froze up tighter than a wedge and it's gonna be a while before I can try it out. Thanks for the help, JPS

 

[hondon]

Re: Mounting height for jet drive?

Browndog,if you are not having problems ,keep it where it is.Yours is a tunnel and that changes things substantially.Basically the front of the foot should be very near on plane with the bottom of the boat. Then water test, and adjust from there.Browndogs tunnel means that he can mount his engine even higher and operate in more shallow conditions.

 

[dick]

Re: Mounting height for jet drive?

Browndog <br />If you don't ventilate in turns and the boat performs well leave it.All you will do is create spray and loose top speed by lowering it.

 

[Oshi]

Re: Mounting height for jet drive?

I know that this is an old post, but hopefully someone will read and respond. I was wondering if you got everything worked out with your boat? THanks<br />oshi