converting to longshaft?

[Johnshan]

I tried a short shaft 35 hp evinrude on my 14' startcraft tri-hull today and once on plane it over-revs and goes slow as death.<br /><br />The boat is rated for a 40 hp and im assuming that I need a long shaft for this boat. <br /><br />Anyway to convert my motor (a mariner 30 hp short shaft) to a long shaft? How much would this cost? Any other way to lower the motor down into the water more? Thanks<br />-John

 

[12Footer]

Re: converting to longshaft?

A couple questions about your setup are needed.;<br />where is the cavitation plate in relationship to the boat's keel?<br />How much freeboard do you require at the transom (cheaper to cut-down transon in some cases).<br />Is the engine using a stock prop?<br /><br /><br />------------------<br />the Early bird get's the worm...So, he don't have to stop for bait.

 

[Johnshan]

Re: converting to longshaft?

the cavitation plate is level with the bottom of the boat, however it does not sit below the V in the bottom.<br /><br />This is a tri-hull so its like this when looking at the back<br /><br />| /\ /\ |<br />| / \ / \ |<br />| / \ / \ |<br />|/ x \ / x \|<br /> \ / <br /> \/<br /><br />Kinda crude drawing but I think u get it. Anyways the plate sits level with the base of the left and right (notice the x, this is where the plate sits. <br /><br />Also the prop we were using was on a 40 hp Johnson before, but we threw it on the 35 hp evinrude for the day since we broke the 35's prop.<br /><br />Now should I try MY motor on this boat? Its a 30 hp mariner and it too sits the same as the other motor. It has a stock prop- and a hydrofoil on it. Any other ideas? Im really discouraged about this boat, it was given to me from a marina near my house, and the whole boat was pretty trashed (rotted floor, seats all ripped, ect ect) so I rebuilt everything, took me about 3 months and now I try it out and it doesnt like the motor. <br /><br />Live and learn I suppose, how fast should a boat like this go? I was assuming that since it was rated for a 40 hp that a 30-35 would move it along pretty fast. <br />Thanks for any comments.<br />-John

 

[Johnshan]

Re: converting to longshaft?

the cavitation plate is level with the bottom of the boat, however it does not sit below the V in the bottom.<br /><br />This is a tri-hull so its like this when looking at the back<br /><br />| /\ /\ |<br />| / \ / \ |<br />| / \ / \ |<br />|/ x \ / x \|<br /> \ / <br /> \/<br /><br />Kinda crude drawing but I think u get it. Anyways the plate sits level with the base of the left and right (notice the x, this is where the plate sits. <br /><br />Also the prop we were using was on a 40 hp Johnson before, but we threw it on the 35 hp evinrude for the day since we broke the 35's prop.<br /><br />Now should I try MY motor on this boat? Its a 30 hp mariner and it too sits the same as the other motor. It has a stock prop- and a hydrofoil on it. Any other ideas? Im really discouraged about this boat, it was given to me from a marina near my house, and the whole boat was pretty trashed (rotted floor, seats all ripped, ect ect) so I rebuilt everything, took me about 3 months and now I try it out and it doesnt like the motor. <br /><br />Live and learn I suppose, how fast should a boat like this go? I was assuming that since it was rated for a 40 hp that a 30-35 would move it along pretty fast. <br />Thanks for any comments.<br />-John

 

[Johnshan]

Re: converting to longshaft?

well that drawing didnt work sorry<br />-John

 

[12Footer]

Re: converting to longshaft?

No it didn't werk, but nice try,and I DID get the meaning.So it isn't set even with the bottom of the Vee, and does need to be lowered. However,on a boat that size, it would still aproach planing speed,but not quite,and would cavitiate.<br />Have you considered cutting-down the transom? It appears you would only have to drop it a few inches.<br />If you cant get faster than a fast idle speed outa the boat, I would suspect other problems,such as a spun hub in the prop.<br />Yu asked how fast it should be expected to go,I would guess 25-35MPH at WOT, and would take a good while to reach plane,even at max rated HP,because of the boat's size and presumed wieght.<br />Can the 40HP prop deliver maximum rated RPM?<br />I'm still thinin prop here,sorry bout that.<br /><br /><br />------------------<br />the Early bird get's the worm...So, he don't have to stop for bait.

 

[12Footer]

Re: converting to longshaft?

Still thinkin prop, but trying that 30HP motor might tell you alot.<br /><br /><br /><br />------------------<br />the Early bird get's the worm...So, he don't have to stop for bait.

 

[12Footer]

Re: converting to longshaft?

Outboards are the same from short and long shaft. The difference is in the length of the lower unit. They are expensive,around 500<br />(used and condition unknown),to 1500 (rebuilt).<br />Sorry about the post-on-post, but we can't edit or change anything once we hit that "submit reply" tab

<br /><br /><br />------------------<br />the Early bird get's the worm...So, he don't have to stop for bait.

 

[Johnshan]

Re: converting to longshaft?

Well it does get on plane, but once it does the motor over - revs due to lack of water.<br /><br />If I move the motor off to the side of the transom, where the cavitation plate sits more in the water it is fine, and acts normal, speeds seem like 15 - 20 mph.<br /><br />The boat actually plains out really quickly when the motor sits more in the water. its a pretty light boat, maybe 500 lbs. 14' long 4' wide, center console.<br /><br /> <br />But I think the problem is that the motor doesnt sit in enough water when it planes. Im going to run my 30 hp mariner on this boat, just have it sitting on the right side of the transome, so it sits in the most water possible. I will be able to try this setup this weekend when I get my lower end seals taken care of.<br /><br />Thanks for the info.<br />-John

 

[Johnshan]

Re: converting to longshaft?

By the way, the prop needs to sit 6" lower on the transom. Is this the difference in length between a short shaft and a long shaft? <br /><br />The motor originaly had a short shaft 40 hp on it, so im confused as to why my short shaft doesnt work. I guess the previous owner had the same troubles?<br /><br />Well anyways thanks for the info.<br />-John

 

[12Footer]

Re: converting to longshaft?

Yes, i would think he had the same problems.<br />Another thing you could do,is find a jack plate for it, one that will give you the extra 6". Expensive, but still cheaper than a long shaft L/U.<br /><br /><br />------------------<br />the Early bird get's the worm...So, he don't have to stop for bait.

 

[SeaMasterZ@aol.com]

Re: converting to longshaft?

on the odd chance I ever get a tri hull, couldnt twin engines use the top of the "tunnels" as being the cavitation plate marker?<br /><br />

 

[bhalsdorf]

Re: converting to longshaft?

The difference between a short and long shaft is 5 inches. You can get extension kits to lengthen them. I bought my kit for my 35 hp for $120