Boat Cavitation

[Areleus]

I did a boo boo and would like some help from some experts.

I just recently purchased a 1985 25 horsepower 2 stroke Evinrude and I am very happy, it runs great. The only problem is that it is a short shaft and I have a 20 transom on my jon boat.

An extension kit costs $335 for the part and another $300 for labor. Almost as much as my boat.

I took it to an older man who was a nine time speed boat champion (and engine prepare guy). He made his own boats and created his own pistons. He told me if I like the engine, he would cut the transom down 2 1/2 inches so the caviation plate is parallel with the bottom of the keel. Upon measurement, we only needed 2 1/4 inches to keel.

Please pass along some suggestions?

Thanks,

Areleus

 

[oops!]

Re: Boat Cavitation

search....... backset jackplate

 

[oops!]

Re: Boat Cavitation

and welcome to i boats

 

[Areleus]

Re: Boat Cavitation

search....... backset jackplate

I did search, but nothing substantial.

A little more help, thanks so much.

 

[Areleus]

Re: Boat Cavitation

Friends,

Forgot to mention, the engine is a tiller model.

Once again, I have a 20' transom with a 15' tiller 25 horsepower. Can I do anything with this or should I sell it and get a long shaft.

Were would be the proper height of the engine?

Should the cavitation plate be parallel with the bottom of the boat?

Thanks for the help!

 

[oops!]

Re: Boat Cavitation

im an i/o guy...so im not much help.....its late so the sites slo...by now you would have had 10 answers......

i just remember a post somewhere...that tashasdaddy (one of the moderators) recomended one to someone with a simmelar problem....

lowering the transom wont be good, as water will come into the splash well on de acelleration....the backset jack plate is relatively cheap compaired to other alternatives...it will hang the motor off the back and lower than the transom...i think !

 

[oops!]

Re: Boat Cavitation

yes...the plate is at the same height as the bottom of the hull.

and nuthin wrong selling it and going for twin 150's

it just depends on what you can get for it and what a new (used) one will cost. ....as well as the hassel...

but what do you want?...what i mean is...its your boat...you can do anything you want with it......build a deck and make it a bass boat...raise the freeboard and make a cuddy....!

what is the hp rating?....are you close with that set up?

 

[John_S]

Re: Boat Cavitation

Not sure if you can use these for main drive motor: http://www.iboats.com/Jack_Plates_a...ategory--**********.005539983--view_id.217143 You would have to reach for the tiller.

If you know you are going to keep the boat, trimming the transom down a couple of inches near the motor is not that bad. Otherwise sell, and buy one for your 20" transom.

 

[Dovekie]

Re: Boat Cavitation

I think that it would be safe & practical to have your friend cut the transom down to accomodate your short shaft motor. Depending upon where you want to use the boat it might be wise to modify the splash well (if fitted) or to install one so that you will not have the problem/concern of water coming in over the counter in following sea or quick deceleration. Bear in mind though that you may spend as much $ and perhaps more & certainly as much time to do this, as your estimate ($650) to increase shaft length. Another thing to think about is that if you lower the transom to accomodate your short shaft motor, from then on, you'll be forced to look for a short shaft, unless you again raise the counter.

It might be better to go with leg extension, depending upon your use, planned expenditure on your boat, present condition and future plans to upgrade (boat).

If helpful, I can try to post pictures of the opposite, ie. raising the counter from stock 20" as I hope to do this in reply to another post.
Good luck!

 

[tashasdaddy]

Re: Boat Cavitation

tiller model, cut the transom. you can't operate a tiller using a set back plate dropped for a shot shaft.

 

[beerfilter]

Re: Boat Cavitation

Search EBAY and Craigslist .
These motors are fairly common , and , you should be able to find the spacer or a complete engine with a blown powerhead , for cheap .
Keep any extra parts for spares , or , resell what you don't need .
You might find someone with the opposite problem and work an even swap .

Cutting a transom down would be the LAST option I would consider , but , she's your boat .
Good luck .

 

[Areleus]

Re: Boat Cavitation

Thanks for all the responses you guys! You guys are real pro's and I appreciate it.

One other question, if I am 2 1/4 from the cavitation plate being flush with the keel and I have 2 3/4 space to cut the transom until the tiller is not usable, what do you think????

This guy who I took my engine too, asked me two questions, do you like the motor and do you want me to cut it? He knew exactly what needed to be done just by looking at it as if he had done it before.

What are the unwritten rules about where you want your cavitation plate????

 

[tashasdaddy]

Re: Boat Cavitation

Anti cav plate should be even or up to 1 inch above the keel. i run mine 1" above.

 

[Areleus]

Re: Boat Cavitation

So, I am within 1 1/2 inches of having it be 1 inch above.

what do you think?

 

[beerfilter]

Re: Boat Cavitation

With only 11/2" -2" cut required to put it into the correct location , that sounds like the fastest and most cost effective approach .
That's a lot less than a full 5" chop , so , minimal impact IMO .
You would be setup for an easier repower if that motor dies , as short leg motors are more common .

How much is he charging you to do the surgery , BTW ?

 

[capecanvas]

Re: Boat Cavitation

Another Idea...

If it's not too late...

I have a 25 hp on a 16' grumman with the same problem. I also wanted to seal off the aft deck and motor well so a conventional transom mount wouldn't work. Get a 30 hp four stroke rated auxillary motor bracket.

http://www.garelick.com/obmb.html

The only boat mod is a good backing plate for the bracket and the advantages are many. When I took the boat out the first time it turned out the technically right height for the prop wasn't. I turned two knobs and pulled on the spring loaded handle to adjust by 1" increments till it was perfect. I often use the bracket to adjust height like a jack plate depending on crew / gear / conditions. When deployed your motor is set a foot back so the boat handles and tracks WAY better. When in it's upright position the h2o intakes are just under water and i use it as a launching / skinny water trolling setting. You also get to trailer the motor with the motor in the down position as the braket gets it clear of the road. You do need a tiller extention because of the setback- I made mine telescoping so I can be at the helm in front of my console. The whole setup rules.

 

[Areleus]

Re: Boat Cavitation

He is charging me less than $300.

I am kind of looking at selling the engine and getting a 20. If the engine was designed to be lower, it would have came that way. I am going back and forth on this.

What do you think?

 

[evinice66]

Re: Boat Cavitation

do it yourself buy tools for 100 $ or figure you can sell your motor for $100 less that you bought it for and buy a different higher horse power one for $200 more that you bought yours for..... equalize it out