I have a 39 feet US Navy Seal boat with 2 620hp mercs,Speedmaster 4s,the propepeller shaft is 1 inch below the bottom straight forvard from the shaft,17,5x30 3-blade cleavers,boat weight 13000lb, 1500-1700rpm the prop blows out.What is the next thing to do ???.

Did the boat ever plane? What props were on it?

It these are the original props, ease up to 4000 RPM and see if you can get them to hook up. Try and keep the RPM equal on each engine.

I do not know what the original propellers are,it was only the hull when i bought it,but the original drives are Speedmaster 3 and 580hp,Should i go to 4-blades ???.

Mark,
You could save yourself a lot of pain and expense if you can find out what props were on this boat prior. If this isn't possible, I suggest you work with one of the prop companies
that offer a trial program that would allow you to keep trying props until the props are right.
You not only get an expert to work with, it will be the best value approach to prop your boat.

If this is the boat I am thinking of it originally had 3a drives and several propellers have been tried on this boat. The boat is still manufactured with 3 a drives. Currently it is running Rolla 6 blade props. 5 blade props work well also.

Since you have changed the drives and motors you may very well have to set the boat up again to work with the current set up.

I think the Navy Seal boys won´t tell me anything about the props it is a seacret, but i have read somwhere it was Fountain who made these boats for the navy ?

You might want to contact Randy at Cobra Power out of Miami, FL. He has a program that may work very well for you.

Julie

I think Markku is in Sweden which limits his resources for not only information but parts as well.

Wasn't there someone else on OSO with the same boat with sterlings in it. Classified was painted accross the deck or something. Maybe we could get some help for Markku from this gentleman.

Cougarman

Yes Cougarman i have the same boat as the CLASSIFIED