Transom height

[Jahfreddy]

Hello SportfishingBC!



I have a question about transom height.


I've just purchased a new'ish Mercury SeaPro 25hp long shaft. The motor did not come with a manual, but everything I could find online suggested that this engine (made by Nissan, actually) is a long shaft of 22" and not the usual 20".


Since I am rebuilding the transom on an older 14ft Mirrocraft, I want to find out if I should add 2" to the area where the motor is being mounted.


More specifically, I have 2 questions:

- where is the motor measured from to give you the standard 15" short shaft and 20" long shaft measurements? Is it from the top (inside) of the U-shaped bracket that fits over the transom to the anti-cavitation plate?

- what is the recommended mounting height of a motor in relation to the bottom of the boat at the transom (ie. - the point where each side of the bottom of the boat make a "V" at the bottom of the transom)? Where should the anti-cavitation plate be in relation to the bottom on the boat?



Hopefully these questions aren't too stupid, and as always, many thanks in advance for any and all replies!



Cheers!

 

[cheech]

The leg length is measured from the engine side and inside of the 'u' bracket, down to the cavitation plate. 22" is unusual.
On small boats, I have found it best to have the cavitation plate be in line from the bottom of the keel. If it is too low you get spray coming up the transom. Too high and you can starve the cooling. gl

 

[Jahfreddy]

Thanks for the response Cheech!


Yes, from what I have found online so far, keeping the anti-cavitation plate (sometimes called the anti-ventilation plate) level with the bottom of the keel seems to be most recommended.

Also, yes, 22" is unusual. I came across this website last night, and further down the document it confirms that for certain models of Mercs (my SeaPro 25hp included), the leg lengths are actually longer than that stated. It did confirm that it is 22", and upon measuring the leg as per your suggestion and the website's, my engine does in fact have a 22" leg length.

http://www.marinepartsman.com/installing-an-outboard-on-your-boat.html

I am rebuilding the transom, so thankfully adding that 2" shouldn't be as difficult as a retrofit.



Thanks again for taking the time to reply!



Cheers!

 

[vetteman]

When I wanted to raise my outboard on my aluminum boat I simply added a piece of wood to the top of the transom and then clamped the motor on.This is a quick and easy way to find the best mounting height for your outboard before you make any permanent changes.
Also, I agree with cheech, mounting your O/B with the cavitation plate even with the keel is usually the best route to go.
Dave

 

[Jahfreddy]

Thanks for the reply Vetteman!


Yes, I think I will mount it so the anti-cavitation plate is level with the bottom of the keel.

Another dumb question, if I add a Doel fin/StingRay fin/whaletail to the motor, would that change anything?



Thanks again!

 

[vetteman]

Thanks for the reply Vetteman!


Yes, I think I will mount it so the anti-cavitation plate is level with the bottom of the keel.

Another dumb question, if I add a Doel fin/StingRay fin/whaletail to the motor, would that change anything?



Thanks again!

Some guys love them and swear by them and some guys hate them. I've got friends who claim a better holeshot out of the water but realize a loss of a couple of miles an hour. Other guys claim better holeshot with no loss of top end.
I think that every boat motor combo could act differently to the addition of a fin. It's a try it and see situation.
You have to wonder if a $60.00 piece of plastic can make such a big difference to a motors performance then why don't the O/B manufacterers use them?
Dave

 

[Jahfreddy]

Thanks Vetteman, yeah, I hear ya ...


I have a 12ft'er with a 9.9, and I found that when I was on my own, it had a bit of difficulty getting up on plane. I would try to put more weight in the front end to keep it down and when I added a Doel Fin, I definitely got up on plane more quickly, could stay on plane at (seemingly) lower speeds, and it also helped "round out" turns, so to speak.


I am not going to put one on right away, because I think I have loads of power for a 14ft aluminum, and also because it involves drilling holes in the anti-cavitation plate as well.


Thanks for your comments, they're much appreciated!

 

[Gunsmith]

The biggest difference with a doelfin is stability, I found my boat more stable with one on.
Made a big difference when turning.

 

[cheech]

Some boat/motor combos respond well to the fin. We used to 'extend' our tiller handles to get the bow down.

 

[Jahfreddy]

Thanks Cheech!


I like the tiller extension modification, it sure beats paying for the proprietary ones!



Cheers!

 

[Mi Chica]

With a stern heavy boat, the fin acts like a poor mans trim tabs. It does help get up on the plane and add some stability.

 

[vetteman]

With a stern heavy boat, the fin acts like a poor mans trim tabs. It does help get up on the plane and add some stability.

It's a shame that you have to drill the cavitation plate to mount a fin to see how it works for your own application. I'd love to try one on my lake boat. When I had my 2 stroke 20HP outboard on it you could almost get thrown out of the boat it had so much torque. When I replaced it with a 4 stroke I had to lean as far forward as possible and the boat still struggled to get up on plane. I've repitched the prop a couple of times and now the performance can best be described as adequate and nothing more. I don't really want to chance losing any top end speed but then neither do I want holes left in the cavitation plate if the fin does not meet my expectations.
Dave

 

[TheBigGuy]

There are several models of fins that don't require drilling. I can't remember which one's, google it and I'm sure you'll find them.

 

[cheech]

Don't worry about drilling into cav plate. When you are done simply fill with JB Weld.
It almost matches the color perfectly. No structural degradation either.

 

[vetteman]

Don't worry about drilling into cav plate. When you are done simply fill with JB Weld.
It almost matches the color perfectly. No structural degradation either.

Thanks Cheech. I just may do that. I'm not overly worried about color match(Blue Yamaha) but if need be Yamaha has some pretty good touch up paint. Hopefully if I go to the trouble of mounting a fin I'll be happy with the results and can leave it on.
Dave