Pontoon motor height question

[ThomW]

I know this question has been asked a bunch, but I keep reading different answers, so I figured I put it out here and see what the consensus is. I have a 2001 25 foot Crest tri-toon. It had been running on a 1989 black max 200hp merc. I am finally swapping that motor out for a 2001 Merc 200hp 25" outboard. My question is what height should the motor be mounted at? I have heard some say the cavitation plate should be 1" below bottom of middle pontoon, others say lower, and some say higher--like the middle of the pontoon. I get that the wake coming from pontoons is different than a traditional hull, so where should the prop and/or cavitation plate be in relation to the middle pontoon? Thanks guys!

Also, the old motor was running a 15 pitch prop...seemed to run well, about the RPM's I needed. Is that a good pitch for this set up, or should I try something else?

 

[snowseeker]

With my motor swap I am starting right around 1" below middle and will test. If the prop cavitates I will lower it another hole and test again. I like to run motors as high as possible.

 

[ahicks]

Level to 1" below is pretty much what I've been taught. Then test and adjust as needed.

The right pitch is about the engine turning full rated rpm at wide open throttle. For performance applications, you generally want something that will turn up towards the top rated rpm.

 

[ThomW]

Just to be clear, level to 1" below the bottom of the middle pontoon?
Or will that be affected by the way the boat sits in the water? What I mean is if the rear sits low in the water, and the water line is say 1/2 way up the toons in the rear, should I aim to have the plate be 1" below the water line? Thanks for the help guys!

 

[snowseeker]

Center line of the pontoons themselves. So if you found the center (top to bottom) of each pontoon and taped a string across, the cavi plate would sit just below that string by around an inch.

 

[ThomW]

Thanks snowseeker. This is the same for a tri-toon as well, correct?

 

[ahicks]

So we have a difference of opinion. I would use the bottom of the center 'toon or engine pod as my reference point.

 

[snowseeker]

If it is a tri toon I as well would use the bottom just the same as a normal boat to set motor height. But I do know some of the "engine pods" as mentioned can ride out of the water when on plane.

 

[Sea Rider]

Can you post a side pic of lower leg showing both upper and lower horizontal plates ? will that toon fully plane or it's just good for fast displacement speeds on calm flat water cond ?

Happy Boating

 

[ThomW]

I'm a little confused. I've never heard of an "engine pod" What is that? Is that a term for where the motor mounts on the pontoon boat?

What do you mean will that toon fully plane? The third pontoon runs almost the full length of the boat. It starts even with the other two pontoons at the front of the boat and I believe it stops a few feet short of the outside pontoons at the rear of the boat. The new motor is not mounted yet, so I can't take a picture of it yet. Wanted an idea of the mounting height before I started, since I have to travel to use an engine lift at a friend's for this job! Thanks for all the input and help guys...really appreciate it!

 

[ahicks]

You have it. Pod = engine mount.

Because most pontoons are displacement hulls, some delight in saying they will not plane. Many that own and use them know differently, though I have to admit it's not a very "clean" plane, so I generally don't argue the point. When you add strakes to a 'toon, in my mind you've transformed a displacement type hull into a planing type hull, confusing things even further in the minds of those that like to argue this point. Let them argue the point. When somebody says it's planing, most understand what's happening.

Engine pods are not universally made. Their "depth" can vary as compared to the back of the pontoons, so snowseeker's point is a good one. My experience has been that most newer boats will run with the bottom of the engine pod solidly engaged with the surface of the water when on plane, though I've been around long enough to know anything is possible - especially with older boats.

You're playing with a tri-toon, so why don't we just stay with that? Have you done any measuring to see how well this new 25" engine is going to line up on a boat set up for a 20" engine? Getting that to work could be a real treat, requiring the transom be modified to carry the engine 5" higher than designed. Not knowing what might be involved there, it might be worth mentioning that converting the engine from a 25" to a 20" is often possible depending on the engine. Which would be cheaper to modify is anyone's guess, especially if you could locate used engine parts to do the conversion from a mechanic that specializes on your engine.

There's the "compromise" thought as well. Like how much performance would you be leaving on the table if you mounted your 25" engine as high as possible on a 20" transom. That might be worth checking into as well. I have no clue....

 

[Sea Rider]

If 25" OB is mounted on a 20" engine mount will surely produce severe back water splah towars mount or over OB's middle leg as water flow at speed will be hitting hard the non edge border of the lower leg portion in which water flow is cut nicely.

If mounted direct on a standard transom much worse. Seems that will need to buy/fabricate same engine mount 5" taller to compensate proper OB height.. Post a pic of the engine mount shot straight.

Happy Boating

 

[darrklim2]

No Title

A picture of the back of the pontoon would be a good starting point even without the engine there. One picture level with the bottom of the transom looking forward like this

and one diagnal like this but from a lower more level angle.

 

[MH Hawker]

when i did my re power i started with it all the way down, then worked with props to get the rpm right in clean water, then started raising it, in my case all the way down was best

 

[ThomW]

Thanks for all the information guys.

I am 99% sure that the old 1987 Black Max 200HP motor that was on my pontoon before was a 25" shaft. It was an "EXLPTO" model, which I assumed meant it was a 25" length (the EXL part). It had the extra "spacing" section that sat between the gear housing of the lower unit and the drive shaft housing. Probably an 4-5 inch additional section? It mounted fine, and was actually on the top holes of motor (meaning it could be raised if needed). I had no issues with that motor being too low or causing excess drag or back splash.

Ahicks--how would I know for sure if my pontoon is set up for a 20" or 25" motor? As I said, I assumed--and perhaps mistakenly--that the previous engine was in fact a 25" motor, so that what I went with this time. I will take some measurements tonight and get some pics to post. Thanks again guys, really appreciate it. Although now I am kinda freaking out that I bought the wrong length motor for my use! :faint2:

 

[ahicks]

That's a darn good question, and I don't have a very good answer! Because you didn't mention the length of the original engine I just assumed you had a 20". The only place I've seen 25" is on offshore boats, but that may be because I haven't spent any time around tri-toons - so don't freak out!

I just went out and had a look at the 'toon I'm rebuilding. That engine pod measures about 17" top to bottom. I tried to get my trusty old 40hp, which I know to be a 20", to tell me why it's a 20", and I can't get it to tell me that no matter how I measure. I would have thought it would be 20" from the cavitation plate to the top of the engine mount, but that's not right either! It's only about 19", which is about the same as the 90hp Honda that's replacing it. Maybe you can use some of those dims to comfort yourself.

 

[ThomW]

Haha. I will take some comfort in your uncertainty! It was pouring rain all night so I didn't get out to take any pictures of the set up. I will measure my "pod" and see how it compares to the one you have at 17". Either way, I am going to attempt the new motor mount tomorrow afternoon, so I'll let you know how it goes...or doesn't go! As long as I know I am trying to get cavitation plate about an inch below the pod and/o center toon, I will start with that and see what the motor and mount allow for adjustments. Thanks again for the help and uncertain assurance that it'll all be alright!

 

[snowseeker]

There is this diagram here: http://www.boatmotors.com/ref/shaft_length.html

Might be of some help.

 

[ahicks]

That diagram is pretty much what I had in the back of my head/expected to see - but nope! Maybe the tape measure shrunk or something (raining here too) - I even had my glasses on!

 

[ThomW]

Haha...Yeah. The diagram would seem logical, but according to Ahicks measuring, it doesn't quite pan out on the pontoons. Maybe a standard transom is different than the pods on pontoons. I'll measure my pod tonight and see what she measures up to. If things don't work out with the mount tomorrow, I'll have a nice 2001 Merc 200 HP motor for sale!