Motor height question

[Muzza]

I have a 12' inflatable with a 15" transom. I bought a 1974 Johnson 25
short shaft motor for it. The trouble is the shaft is not that short...about
17.5" from the bottom of the mounting clamps to the bottom of the
cavitation plate. I have ordered a "mini Jacker" which bolts to the transom
and can raise the motor up to 3". What is the optimum motor height?
Do you line up the cavitation plate with the bottom of the transom or should
it be lined up with the bottom of the tubes? Any help or advice would be greatly appreciated.

 

[tashasdaddy]

Re: Motor height question

that motor has the old lower unit, and needs to be that deep, to maintain cooling water pick up. the older motor short shafts were 17 inches. long were 22 inches. new ones are redesigned and are only 15 and 20 inches.

 

[Muzza]

Re: Motor height question

Tashasdaddy
Thanks for the quick reply.Sounds like I need to leave things as they are.
It's a shame tho', I was expecting better performance from this motor and I thought that raising it would help out. Is it safe to bring it up 1" or am I looking for trouble?
Thanks

 

[mattyv]

Re: Motor height question

try it out: make sure it picks up water and doesn't cavitate. You probably won't lose much performance from an extra inch sticking down

 

[Muzza]

Re: Motor height question

I raised the motor 1" with a block of wood on the transom this weekend.
The boat was great in the 5mph zone, planing at low speeds. The second I
gave it some throttle the was massive cavitation. Raising the motor is obviously not the answer. I've now added an SE 200 Hydrofoil which has helped performance but I'm still experiencing the cavitation or whatever it is.
This is driving me insane...the more I read the more possibilities. Maybe it's a spun hub? bad prop? or I need a true 15" shaft? Maybe I should take up Golf.
Any help would be appreciated.
Muzza

 

[TOHATSU GURU]

Re: Motor height question

All modern small outboards are 16" to 16 1/2" in the short shaft and all of them require the use of a 1" block on top of an inflatable's transom to get close to designed performance. Try tucking the engine in one more hole on the trim adjustment.

 

[jnewtonsem]

Re: Motor height question

Elvin- Why do you say all inflatables require a 1" block? And what performance difference frm sitting directly on the transom? It would seem sitting directly on the transom would pose less risk of losing motor if it is not bolted on.

 

[TOHATSU GURU]

Re: Motor height question

Risk aside...Boat manufacturers still use the 15" transom in most small boat designs and back when time began most outboard manufacturers did too. But over the last 25 years there have been changes to industry standards in outboards that boat manufacturers have not adapted to. All Japanese short shaft outboards are too long for a 15" transom...If you are looking to maximize performance. Inflatables tend to run best when the cavitation plate of the engine is above the bottom of the boat anywhere from 1/2" to 1 1/2'.
This is a perfomance issue rather than a safety issue. A poor analogy, but you get the idea, would be higher air pressure in your tires than the car manufacturer recommends. You get better MPH but, you might lose cornering ability. I asked a friend of mine a few years ago, that owns an inflatable boat company, why he has never changed his transom height and got the following response:

"There are a lot of people in the 3rd *******world who still run old engines that are 15" short shafts. If I changed the ******* design they would burn their ******* piece of **** engines up and blame me."

As too what the advantages are:

Increased speed at top end
Reduced time to plane
Reduced back spray coming over the transom
Reduced risk of damage to the engine from grounding

Disadvantages:

None, if done within reason.

 

[Nos4r2]

Re: Motor height question

I raised the motor 1" with a block of wood on the transom this weekend.
The boat was great in the 5mph zone, planing at low speeds. The second I
gave it some throttle the was massive cavitation. Raising the motor is obviously not the answer. I've now added an SE 200 Hydrofoil which has helped performance but I'm still experiencing the cavitation or whatever it is.
This is driving me insane...the more I read the more possibilities. Maybe it's a spun hub? bad prop? or I need a true 15" shaft? Maybe I should take up Golf.
Any help would be appreciated.
Muzza

I had this problem too on a sib. Make sure the keel is pumped up hard-also try sitting on the floor and see if it improves.

 

[mudslinging79]

Re: Motor height question

i have the same setup only with an 18 horse, i notice that it seems to be too long, until i get up on a plane. then it appears the water pickup is almost at the surface. i would leave it as it is stock, have you played with the trim pin? i found out i had to adjust for the choppy water vs the calm water, if its choppy then i run the trim pin in the bottom hole, calm water im good no more than the second setting, much more and the prop captivates when turning sharp when planing. and yes, sitting on the floor in the back helps.

 

[Muzza]

Re: Motor height question

2 Words....trim pin. Thank you. I took it out of Dana Point harbor this
afternoon. Had more fun than should be legal. I gave up before the motor
did. WOT is out of control literally. Planing beautifully, cornering insane.
My first boat ,my first solo run...awesome. Frustrating start but I but I know
why I bought it now.Thanks everyone for your input ...great forum
I'll go and enjoy my boat now. I think I need cusions for the seat..what a pounding.
Muzza