Porpoising on 98 gulfstream with repower

[LongIslandGulfstream]

Ok so long story short I used to have a 250ox66 on my 98 gulfstream. After a few seasons I swapped a 4blade powertech on it and the boat rode exponentially better than the stock 3 blade. I was so happy with the stern lift which made the boat float over the chop like a Cadillac compared to getting a pounding out there on some days. As everyone knows these hulls are not made to run at high speed. I usually cruise between 26-30 mph, and that’s all the conditions usually allow.

fast forward I repowered with a 2023 suzuki df300 with the stock 3 blade prop. It weighs approx 140lbs more and it is mounted one hole higher than my ox66 was. The cavitation plate is justttttt below the surface of the water when on plane and trimmed out that I think going up another hole would be too much. On any significant chop or crossing a large wake I experience porpoising that is to the point uncomfortable. Trimming down does not necessarily solve the problem but it does help a bit. At rest, I do notice the Grady drive bracket is sitting lower in the water with the additional weight of the new motor, approximately 2 inches. I am trying to find the right 4 blade prop right now but In the meantime I’m going to experiment with adding 100lbs of sand to the anchor locker to see if that helps. Anyone else have a similar issue?

thanks

 

[PointedRose]

Did you use tabs with the old motor and do you with the new motor?

 

[LongIslandGulfstream]

Never needed to use tabs with the old motor (I used them 90% of the time to correct side to side lean). Using tabs with the new motor helps but doesn’t solve the porpoising. Today I actually just stuffed 150lbs of pea gravel in the anchor locker/front of cabin and the boat did run a lot better. While docked the motor and bracket are at least 2 inches higher. Waiting for a choppy day to put it through its paces but all signs are promising thus far. I’m beginning to think the original 1998 Grady bracket was not designed for even the 600lb limit that Grady white themselves say it is spec’d for. Times were a lot different back then I guess.

 

[Ekea]

call ken at prop gods to help find the right prop.

also, you are saying that addint 150lb (one person worth) of weight to the anchor locker brought the stern up 2 inches? that seems like WAY more than what makes sense. was your fuel and other weight factors the same in your comparison?

 

[LongIslandGulfstream]

I know it sounds crazy but yes. All other factors remained the same. I simply looked at the water line before adding the weight and after. The motor is like a pendulum all the way out there on the bracket. Yes, I’ve been using Ken for years… great guy. But this suzuki is a different animal tho and much harder to prop correctly on this older boat

 

[dgrizzly]

I went through the same thing when I repowered my GS. Ditched that small pontoon and upgraded to a larger high volume bracket from stainless marine. Jumps on plane at 16kts, I also installed larger trim tabs and believe the extra surface area helps as well. I also lifted the engine up, up, then down to the middle to get the cav plate just below the water. Seems to run a lot better a little buried(less prop slip). I ended up with the power tech LFS4 18 pitch prop and got it tuned locally to add around 300rpm more pitch. Boat now performs better than new. Shoot me a DM if you want to chat 1:1.

 

[232 Gulfstream]

I went through the same thing when I repowered my GS. Ditched that small pontoon and upgraded to a larger high volume bracket from stainless marine. Jumps on plane at 16kts, I also installed larger trim tabs and believe the extra surface area helps as well. I also lifted the engine up, up, then down to the middle to get the cav plate just below the water. Seems to run a lot better a little buried(less prop slip). I ended up with the power tech LFS4 18 pitch prop and got it tuned locally to add around 300rpm more pitch. Boat now performs better than new. Shoot me a DM if you want to chat 1:1.

What kind of performance are you getting with the single 300hp? Top speed? Cruise?