Anti cavitation plate height, should I cut transom

[adrenalizd]

Hello all! I am very new to boating. Beyond green, my previous experience is with my 12 foot on boat with an electric trolling motor.

With that being said, I have been able to trade around and get a 1965 Searay 500 Deluxe as well as a 1969 Mercury merc 350 35hp in perfect shape.

Once I mounted it I have figured that the anti-cavitation plate sits about 4 inches above the bottom of the boat at the closest and at the prop it sits 3 inches above the bottom of the boat. I have included a photo. I have only had the boat out once and was in a lake that was idle speed only so I have no idea of the performance yet but from what I have read the motor is sitting too high.

I have five kids so purchasing a new lower unit or an expensive jack plate are out of the question.
What I am wondering is if the motor will work at that height? If not I will have to cut the transom, I am not sure of the exact procedure of that yet.

Any ideas or opinions will be greatly appreciated.

 

[Sea Rider]

Re: Anti cavitation plate height, should I cut transom

Some tech issues:

Anticav plate/hull's bottom heights stated in owners manual is just a refference from where to start..

1-With boat floating, trim engine to be perpendicular to water lever if that's a non electric model. Your pic shows a trim in setting

2-With at least one heavy boater sitting farthest to bow, go for a wot spin at full plane.

3-Ideal water flow must pass slightly under upper deflector plate (2)

4-Is (1) a round or angle housing part. If at a angle as in (3) water flow can pass under it, if round will have water splashes inside deck.

5-Become a water spotter and check where is water flow passing with respect to leg, that way will know if in need to rise or chop transom down for best boat/engine water performance.

6-You should aim to achieve something like this...(second pic)

7-Check modified pic for better understanding...

Happy Boating

 

[coolbri70]

Re: Anti cavitation plate height, should I cut transom

the problem with cutting down the transom is the water level may come over when you come off a plane your wake may swamp yor boat

 

[adrenalizd]

Re: Anti cavitation plate height, should I cut transom

Thank you Sea Rider and Cool Bri!!


I will reads up on how to set the trim, Like I said I am very new to this...lol

Thank you so much for taking the time to draw on the photo, that actually clears things up quite a bit!!

Number 1 is an angled part.

When I measured the shaft length it comes up as 17", an odd size I know, and my transom is 20".

I will have to take it out to a different lake about two hours from here to be able to check in water. I have a lake about 30 minutes away but it is idle speed only. That is where my son and I took it last Saturday. It handled great and drove wonderfully but then again we were only at idle speed. The boat sits pretty low in the water, where it has a v-shaped hull, Even though the cav plate is 3 to 4 inches higher than the bottom of the boat at the center, it is dead even with the bottom of the boat on both sides.

Again thank you guys!!! you are great, my kids are looking forward to going out in it. I am not sure what speed to expect, the boat weighs 650 lbs and the motor is a 35hp. We mainly will be using it for fishing. I traded for the engine and believe it will handle all of the lakes and rivers around here, central and northern Kentucky. Hopefully it will even be fast enough to do a little tubing.

 

[ondarvr]

Re: Anti cavitation plate height, should I cut transom

The shaft length is too short for your boat, it won't perform well at much more than an idle the way it is now. It may be possible to cut the transom down to fit the motor, but there may be some possibly serious issues if you do that.

Like has been stated, the boat was designed for use with a higher transom, lowering it may cause water to come in over the transom. This could be a serious problem or an inconvenience, it's hard to say without seeing more of the transom.

Some boats have a large splash well that can handle a good deal of water, others don't.

 

[adrenalizd]

Re: Anti cavitation plate height, should I cut transom

The shaft length is too short for your boat, it won't perform well at much more than an idle the way it is now. It may be possible to cut the transom down to fit the motor, but there may be some possibly serious issues if you do that.

Like has been stated, the boat was designed for use with a higher transom, lowering it may cause water to come in over the transom. This could be a serious problem or an inconvenience, it's hard to say without seeing more of the transom.

Some boats have a large splash well that can handle a good deal of water, others don't.

Thank you ondarvr!!
Here are some more pictures to give you a better idea.

 

[Sea Rider]

Re: Anti cavitation plate height, should I cut transom

If it's a manual trim, simply pivot and lock engine upward, release trust rod lock and place in hole that will give a perpendicular engine with respect to sea level, generally should be 2-3 holes out transom.

The only way to know if in need to raise transom or chop it down a bit, would be to to sit on deck next to engine, pull your neck a bit outside transom and check leg/water flow, it's a must test at full plane on flat glassy no wind waters.

Have chopped transoms down and rise transoms as well on new boats using up to 50 HP engines, all are performing excellent with owner's content. You should not come to a complete stop after being at full plane, worst at wot, must throttle down in short steps to shorten the wake, if not, the big wake will probably jump over transom...

Happy Boating

 

[adrenalizd]

Re: Anti cavitation plate height, should I cut transom

Thank you Sea Rider!! I just went out and checked, I will have to pick up a pin for the trim as there isn't one on the motor. I will hopefully be able to test it soon. Thank you so much!!

If it's a manual trim, simply pivot and lock engine upward, release trust rod lock and place in hole that will give a perpendicular engine with respect to sea level, generally should be 2-3 holes out transom.

The only way to know if in need to raise transom or chop it down a bit, would be to to sit on deck next to engine, pull your neck a bit outside transom and check leg/water flow, it's a must test at full plane on flat glassy no wind waters.

Have chopped transoms down and rise transoms as well on new boats using up to 50 HP engines, all are performing excellent with owner's content. You should not come to a complete stop after being at full plane, worst at wot, must throttle down in short steps to shorten the wake, if not, the big wake will probably jump over transom...

Happy Boating

 

[Texasmark]

Re: Anti cavitation plate height, should I cut transom

Sir you have a short shaft engine on a long shaft (required) boat. The transom height for that engine is 15" even though the engine is a tad longer. That's another story.

If you cut the transom down, you could be destroying structure and your splash well will go to pot and you would really need one with a 15" transom. It is really hard to keep following seas and coming off plane water out of the boat with a 15. That's why the industry upped the anti to 20 and in recent years 30 and maybe 35 for offshore applications. I guarantee you that extra 5" makes a whopping difference in a dry boat.

Bad idea. Get a long shaft mid section requiring a longer drive shaft, water tube, and shift shaft, another engine, or another boat.

HTH,
Mark

 

[adrenalizd]

Re: Anti cavitation plate height, should I cut transom

Thank you for the response TexasMark. Unfortunately I have no cash what so ever. I am trying to figure out an option with what I have. Is there a risk of it overheating, or is it just a steering problem, etc. with the shorter shaft?
I would love to be able to make it longer but it seems that the parts to do it are going to cost quite a bit, if I can even find them. It is a 1969 Mercury Merc 350 35 hp.

Like I said I have zero cash left, i just spent the last I had ordering new control cables for the motor. I am really hoping to be able to take the kids out, they are going to be super disappointed if we can't.

 

[82rude]

Re: Anti cavitation plate height, should I cut transom

see if you can trade somebody for a long shaft motor though ive had sucess cutting down transoms its not for newbies (no offence intended).as for the kids being dissappointed better that than maybe drowning or sinking .

 

[ondarvr]

Re: Anti cavitation plate height, should I cut transom

Your boat appears to have a rather large splash well, so it may be possible to cut the transom down, not that I am recommending it though. You will still need to spend money on supplies to do it correctly, plus you may find some rotten wood that will need replacing, so expect to spend some money.

 

[adrenalizd]

Re: Anti cavitation plate height, should I cut transom

Thank you all!! I really appreciate you all taking time to help me with this!

I am really beginning to hate boats though. I bought a 5hp johnson for the jonboat to take the kids fishing. It was running when I went and got it. Put it on the boat and it wouldn't start. A year later, new points, rebuilt carb etc. and it still won't run. I was able to pick this boat up for $200 even though I had to drive twelve hours to get it, then was able to trade some things that I had and then drive another 18 hours to get the motor. Put new steering cables in and ordered new control cables for the motor, only to figure out that there is a shaft difference. I had researched motors but didn't realize until it was too late that there is a shaft length difference. It was my fault for not realizing that but at the same time I am about to give up on boating altogether.

 

[adrenalizd]

Re: Anti cavitation plate height, should I cut transom

My thought on it now is that I know that it runs great at idle speed so it will still work great as a fishing boat. Hopefully by this time next year I can buy the kit to make it a 20 inch motor and see It's potential. At least we have a boat that floats and moves. Baby steps, baby steps...lol

 

[jbjennings]

Re: Anti cavitation plate height, should I cut transom

Cut your losses and sell it. Old mercs and old fiberglass boats are not for folks new to boating.
Having a reliable boat/motor/trailer that you can fish out of and pull a tube with multiple passengers is not an inexpensive venture.
If you have no skills at working on boats/outboards, you really want something relatively new. A qualified mechanic costs too much to make repairing an outboard that old worth it.
JMO,
JBJ

.

 

[jbjennings]

Re: Anti cavitation plate height, should I cut transom

aomci.org's free classified ad section would be a good spot to look for parts to make it a long shaft. I'd look closely at the wiring harness on that mercury.

 

[roscoe]

Re: Anti cavitation plate height, should I cut transom

My thought on it now is that I know that it runs great at idle speed so it will still work great as a fishing boat. Hopefully by this time next year I can buy the kit to make it a 20 inch motor and see It's potential. At least we have a boat that floats and moves. Baby steps, baby steps...lol

Love your attitude.
Baby steps for sure, thats fine.
We all have to start somewhere.

The problem with the short motor.... is that is will not develop enough thrust with the prop hidden behind the boat like that.

As long as the lower unit is under water so the engine get cooling water, you can run it.
You probably will not get the boat up on plane the way it is now, but you can push the boat around at 5-7 mph.
This is called displacement speed, or hull speed, the fastest it will go while plowing through the water instead of up on top of the water (on plane).

So, once the boat is up to this speed, 5, 7, 9 mph, then don't push it any further,
as it will just strain the motor, (and look silly ).

Good luck fishing, happy boating, and enjoy the time with the kids.

 

[coolbri70]

Re: Anti cavitation plate height, should I cut transom

you will be able to putt, at displacement speed, but if the boat would plane it would lift your prop out of the water, it would lose thrust and come back down, if you keep looking, you may find that someone has a long shaft on their 15" transom, who would be happy to trade you, solving both your problems. i know i have seen threads on here by folks with the opposite problem, their prop is too deep:facepalm:





'

 

[Texasmark]

Re: Anti cavitation plate height, should I cut transom

Love your attitude.
Baby steps for sure, thats fine.
We all have to start somewhere.

The problem with the short motor.... is that is will not develop enough thrust with the prop hidden behind the boat like that.

As long as the lower unit is under water so the engine get cooling water, you can run it.
You probably will not get the boat up on plane the way it is now, but you can push the boat around at 5-7 mph.
This is called displacement speed, or hull speed, the fastest it will go while plowing through the water instead of up on top of the water (on plane).

So, once the boat is up to this speed, 5, 7, 9 mph, then don't push it any further,
as it will just strain the motor, (and look silly ).

Good luck fishing, happy boating, and enjoy the time with the kids.

+1 or Ditto, whichever you understand. The prop has to see new water to operate properly and the rows of holes along the lower unit are the water pickup holes which are required to be under water at all times when operating. Easiest way to ascertain that is to get in front of the boat and look back under the hull. If you can't see all of the prop (unless you have a bass boat with setback and all) it can't see new water.

On cutting down with a deep splash well, think about tilting the engine up. Cables and the engine both have no place to go if it is right beneath the clamp.

Do as I suggested earlier!!!!!

Mark

 

[lexer440]

Re: Anti cavitation plate height, should I cut transom

A few years ago I had a fibreglass boat with a large splashwell and the right outboard on it. In a following sea at idle the splashwell got used, when I came down off plane it got used sometimes too, I would not like to think about cutting 5" off the protection offered by that. The splashwell may get more water than it was designed to cope with.