Stern heavy

[CedarCam]

I have a 1988 Sunrunner (cuddy) that was 18 feet I/O but has had a pod added and converted to an outboard. Powering it is a 1999 150 HP Evinrude Ficht Ram with a 3 blade aluminum prop (15x15). I have no idea what the WOT is because the tach isn't working properly (one of a few bugs I need to work out with this boat). The problem is that when there are 2 of us in the boat running at 25 knots or higher, it doesn't take much chop to cause us to start bouncing pretty hard (really fly and slam hitting even a light wake). This is with the motor all the way down. It also loses it's plane fairly quickly as I power down. From what I have read, a stern raising (4 blade) prop should fix this problem for us. What pitch should I be looking at? Or can I even ask that question with out knowing what my WOT rpm's are?

Thanks

 

[steelespike]

Re: Stern heavy

I have to wonder if the foam under the floor may have absorbed water over the years.
It doesn't seem to me that a 150 and bracket would weigh any more than an I/O setup.
Yes a 4 blade should help the problem but I think a check of the condition of the foam
first.
If the transom angle isn't enough the motor can't tuck in as far as it should ,would cause the bow to be high,bounce and fall off plane easily.You can buy or make wedges to improve the motor angle.
If the hull was rebuilt the bottom may have too much rocker.
Can't be sure but a 15" prop seems a little small for an 18 footer could be an indication of too much weight.(water in the foam)

 

[CedarCam]

Re: Stern heavy

It's not that the boat is necessarily heavier, but the conversion from I/O to outboard has moved the weight of the motor about 4 feet farther back, changing the distribution. Will see if I can ad the pic to the post

 

[craze1cars]

Re: Stern heavy

This's a very interesting Frankenboat....

I find it very odd that this "pod" that was created on the back allows the hull to essentially slope upward as it approaches the motor. Not to mention being narrower than the boat. I very much suspect that the pod setup and design, which is essentially reshaping the entire hull of the boat at its most critical location, is causing your porpoising. And I highly doubt that a stern lifting prop will correct it...but I suppose it might...this is uncharted territory here I think.

Trim tabs would certainly help tremendously, however I have no idea if that boat has a place to install them anymore...

It is not possible for anyone to suggest a pitch if you don't know your RPMs.

 

[CedarCam]

Re: Stern heavy

Thanks...I was concerned about the rpm issue...will resolve that this week. Podded boats like this are actually quite common around here. I assume that a lot of people buy I/O's and then decide they would rather have straight outboards.

I think there is lots of room for trim tabs or smart tabs...will investigate adding those.

 

[craze1cars]

Re: Stern heavy

Agreed. If there is room to place either trim tabs or smart tabs on that boat...either alongside the pod, or on the pod (I have no idea which might be better), I am sure that would be your most effective solution to the porpoising problem.

Snap a pic of the back of your boat and send it to a tab manufacturer, and they can suggest your placement and product options.

 

[SparkieBoat]

Re: Stern heavy

trim tabs...get you a set

 

[CedarCam]

Re: Stern heavy

Just to update this thread, I couldn't afford the real trim tabs, so I threw a set of smart tabs on and I can't believe the difference. I could barely do 15 mph before without porpising and bouncing around...now 30 mph (more if I want) is no problem. Thanks for the help. I still want to throw a 4 blade prop on so next I will get my tach working properly so that I can figure out my rpm at wot.

Cheers

 

[steelespike]

Re: Stern heavy

Very likely if you go to the 4 you will need to change the tab setting after you see how it runs.
Also if the tabs are set to strong the boat may plow and steer strangely especially in a following sea.From your description it does sound like you have great improvement.