B25 Sport Convertible Outboard Height Adjustment

[cmass494]

I’ve been enjoying my 1963 25’ Moppie Sport Convertible which was converted to an OB bracket with a 300hp Suzuki. I’m contemplating raising my OB up one or two holes on the bracket to potentially fix a few issues. First is when I accelerate I get up plane fine, but before I do the OB splashes a ton of water straight up over OB and swim platform. Kind of annoying. Second to achieve my best performance and top speed I have to trim my OB up to 72% on my gauge. That seems like a lot to me. It starts to reduce speed once I go over 72%. Third I would like to get my bow up higher in rougher water to prevent the pounding at mid speed range. I’ve read the 25’ DV sits much further back and if I can get me bow up higher, then it wouldn’t pound as hard. Fourth I would like to see if I can stay on plane at slower speeds. Currently I have to be at 4000 rpms and 25 mph to stay on plane. Would be great if I could stay on plane at slower speeds to conserve fuel and just cruise.

So the big questions would be by raising the OB would it solve for some of above, what will I give up beside potentially losing torque to get up on plane and how many holes should I go up (I have two holes left to go up)? Another thought was to put more weight in the Stern to raise my bow? Someone told me to fill my OB bracket with water to add more weight. Concerned with too much weight on transom with the OB and water.

Welcome your thoughts on what I should do or if I should just leave it as is.

Thanks! Chris

[ktm_2000]

Hi Chris,

something's off... When I had the twin 135hp I/Os, I did a test shutting one motor down and trimming up and was able to get the boat up on plane doing @17kts on one motor. That 300 zuki should have more than enough power to keep you on plane at a lower speed.

When the boat is out of the water, if you drew an imaginary line following the keel of the boat back to where the motor sits, how high off that line is the anti-ventilation plate? Without the bracket the anti-ventilation plate should be about even with that imaginary line. I seem to remember that you have a bracket so I believe the standard for motor height is 1" up for every 12" back off the transom. (please research this before making any changes)

your other comment about running bow down confuses me, my boat would run bow proud and flatten out as I went faster, always with some bow lift though, what attitude does it run at?
here are some pics of my boat running
slow just barely on plane, 12-14kts
https://photos.app.goo.gl/Z2a7HcwXdgV5NrGk9

guess at speed @20-22kts
https://photos.app.goo.gl/HcetSqFH4g9Q8fvG6
https://photos.app.goo.gl/sqPdsqoFFL7CsP7U9

closer to 30kts
https://photos.app.goo.gl/N8PY7JNDdDmQdir27

[Yannis]

Matt,

You have considerably more weight in the back with two I/O's, than with an OB.

[Waytooslow]

How about a picture of your set up out of the water? There is plenty going on that not making much sense.

[ktm_2000]

I'm curious here because I will have a bracket and most likely end up with a 300hp zuki so I would like to understand this for myself as much as helping Chris.

I went searching through Chris's old posts and came up with his google share with photos, this photo somewhat shows the relationship with the anti-ventilation plate and the bottom of the boat when he had the yamaha.
https://photos.app.goo.gl/JtL8qhWT3NV6M4J59

The photo angle is not good for making accurate measurements but my guess is that the anti-ventilation plate is about 2-3" above the hull bottom and could go higher, how much? who knows.

Also did the boat perform the same way when you had the yamaha? did it push water up when accelerating and need lots of engine tilt?

[cmass494]

Sorry all I’ve been traveling like crazy for work and haven’t been on boat. Thanks for the responses and questions. Below is a link to some photos of the OB out of the water. It was placed at the same height as my original Yamaha which splashed as well. Will try to get some measurements this weekend of how high above the bottom of the hull the cavitation plate is, as well as, how far back my OB bracket sits. That might tell me how far off it is to your formula above. Don’t get me wrong she runs great with a top speed of about 47 MPH on my GPS. Just want a lower end speed and still stay on plane. More to come!
https://www.amazon.com/photos/shared/Om ... dJY4PxYAgG

[ktm_2000]

looks too low to me, proper measuring will help you out.

Also research the ratio, I did more searching and some say as much as 1" per 5" back for high performance boats (that ratio seems excessive to me).

at least you have 3 more holes to go up before options for raising get ugly so it may be a trial and error thing.

I also saw another post on THT where the person was using a go-pro on a stick to take video of the engine while running, he was demonstrating that his engine was too low because the anti-ventilation plate was under water at cruise speed. If your boat is wet-slipped and a pain to take out, maybe a similar technique could help identify what is going on without pulling the boat.

[Waytooslow]

Is he motor at a running attitude? I ask because that looks like too much bow down trim to me. Height does not look like it is crazy off to me. Nice looking motor. Have you put a straightedge on the last 4 feet of the bottom to look for irregularities and hooks?

[cmass494]

Finally got out on her yesterday and was able to check my OB height. The cavitation plate is actually even with the bottom of the hull. I ran a bar across the keel and out to the OB. The bar was even with the cavitation plate. So looks like I’m just too low. Based on the formula above and my OB bracket extending out 30 inches I need to go up about 2-1/2 inches. I think each hole is about an inch in height so might try going up two holes and see how she does. Open to suggestions! Worried about losing torque to get on plain if I go too high, as well as, if my prop might now be the wrong pitch. Oh well a good place to start! Thanks again for the help!

[Waytooslow]

Sounds like a plan to me.

[ktm_2000]

have you had the opportunity to get the engine raised up? results?

[cmass494]

Sorry all I’ve been traveling for work and unfortunately I don’t expect to raise the OB until mid-August. Will post results as soon as I can get her done. This summer is going by way too fast.

[ktm_2000]

Ouch.... hopefully you are getting a day here or there to get out and runt the boat as is in the mean time.

[cmass494]

So finally had the OB raised two holes and didn’t have that much luck. It seemed fine with getting up on plane, but it quickly shot up to my peak RPMs at WOT. Without trimming it up at all I was at 6100 RPMs. When I did trim it up a little it quickly went over my max range of 6300 RPMS. I decided to drop it down one hole and that seemed to be fine. So at WOT and trimmed up to achieve max speed, I get to about 6100 RPMs at a top speed of 46 MPH. So no gain on top speed, but was able to get her to stay on plane at 3300 RMPs and about 16 MPH. Much better than before where I was at about 4000 RPMs and 22 MPH. Better cruise speed! She still splashes up onto cowl at take off, but a little less than before. My lower unit still sits below the waterline, but again slightly better than before. No real help with the pounding into a headwind, but didn’t have time to really test her out and there weren’t many larger waves. So overall a small improvement, but not exactly what I was hoping for. Oh well I still love my Bertram!!

[ktm_2000]

If the goal was to get the boat to run better at cruise, it seems that you've achieved your goal. It also seems like you achieved the goal of not having to trim up the motor as much to hit WOT.

As for your top end performance, 1 mph loss is not bad to get better mid-range performance. It seems to me that you've got the right prop on to hit max rated RPM range with the motor trimmed normally. I would think that in the past you were trimming the motor up more than you (currently need) to get the prop in the right position to move the boat efficiently. Now that the engine is at a higher height, trimming up is taking load off the prop allowing the engine to spin higher RPMs. If you want to trim up more to get every little bit of performance out of her with the engine in the current position, 400 rpms = 2" of prop pitch, so theoretically you could increase prop pitch, I doubt it would dramatically change things as drag goes up exponentially with speed.

I wouldn't be concerned with it as the speeds you are achieving are quite fast. My experience on my boat has a grand total of maybe 1-2 minutes combined at WOT in 15+ years so take it for what its worth.

I would concentrate at how well she runs at @70% of rated rpm. How does she do around 4000 rpm now? to me the hull rides the best around 24kts (28 mph).

[cmass494]

You are so right! I’m happy with the results and glad for the improved mid range performance. I think she is dialed in and worth raising the one hole. Not at WOT much myself. I will run her again at 4000 RPMs and let you know what I get. My big test will come on a windy day and see how she does into a headwind. Need to find that sweet spot without pounding. So being able to stay on plane at a slower speed will really help.

[Trey Dibrell]

Your motor appears to need raising a good bit.
Also, all 25’s need trim tabs. Boat Leveler Co. makes a Bertram
25 specific trim tab that fits outboard of the transom flat.
They will certainly improve the low speed planning problem
You mention, as well as give better overall performance
Of your boat.