What prop for a stern heavy twin diesel boat
 

We have a 29 foot deep v hull that is very much an offshore type hull. It does have inflatable rubber tubes around it so it doesn't look like an offshore boat but it certainly goes well in the Pacific Ocean. The hull is called a Ribtec 890SX.

It weighs in at about 4400lbs (estimate). The twin diesel Yamaha ME421sti's sterndrives are about 2400lbs for the pair so it is quite stern heavy.

It currently has a pair of Yamaha Hydradrive stainless 3 blade 16 3/8 x 21 props. The drives are Yamaha Hydradrives with a 1.5 ratio. It takes a while for the huge turbos to kick in but when they do, it accelerates like no other 29 foot boat I have ever been in. Gets to the redline surprisingly quickly even with a dozen people on board and all their gear and full tanks.

Tops out at just under 45 knots although have been told it should do just over 50. It is super economical, a day on the water is about 100 litres.

All is great except it doesn't respond to any trim. If I trim the legs out at all it starts to porpoise. It feels as if it needs more stern lift. Adding lots of weight forward lets me use a very slight amount of trim and we go a bit faster.

Most props I can borrow and around the 14 1/2 inch diameter, mainly 4 blades. Any advice on what props I should be trying?

Modifying the propellers you have makes the best sense.
Send them to us.

Brett

I think you are right but I am in Australia and that might be quite difficult.

I have been reading about turning in and out, my port prop spins counter clockwise and starboard clockwise, so I think that means they are turning out. Is that the best way?

I have quite a few pairs of Mach Sterndrive props. Should I try something like a 24 inch prop as they have an extra blade but are smaller in diameter?

You are correct in that is outward rotation.
Try any propellers you have the opportunity to try. I believe the Bravo 1 props done correctly will be the best option.
We do have clients in Australia that send stuff to us so it can be done if you desire.

Brett

Will do some prop testing and report back. Thanks.