A BIG PROBLEM not planning
 

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

This is the boat that I am thinking of. Fountain used to manufacture it. It was then contracted to another builder who still builds it in Mississippi.

I have one of the props in the shop they are currently running and we are working with them now to manufacture propellers for this boat. It will be a few months before we have the prototype finished and then we will begin testing on the boat.

Try extending the bottom cavation fin on the drives 2 or 3 inches. I had the same problem. I have different boat and set up but it worked well for me.

Thanks Dwane i will try that.

By going from 3A's to 4's, you raised the propshaft height about 3inches. I think you'd be luck to get on plane with 6 blades, that propshaft height and that much weight. Not to mention, your propslip will probably be very high.

Do anybody have a good guess what will happend if i try 17,5x28 4-blade cleavers i can get them pretty cheap.

Is there a reason you don't want to go back to 3A's and get the drive back in the water for your hull set-up ?

Have any pictures of your boat ?

Cougarman

Markku you know the 4,s are alot shorter then the 3,s. did you mount the gimbles in the same hole ? My 30 super was hard to plane off. I had MRD make a custom cavitation plate for the drive and that took plaining rpm down to 3500 then added a dolphin ,looks kind of funny but the rpm is under 3000 to get it on plane . My boats for sale in the superboat forum maybe a picture will help you see what it looks like. check it out. Griffs rite on the money. You need more blades very pricey. I bet 2 inch spacers would help greatly. They can be had fairly cheap.

Yes here are some pictures

spacers, 4 bld's, and a cavplate similiar to the one used with imco shorties just designed for the 4 will work well. i had a similiar issue when rerigging an old sonic from 3's to 4's.

Are you using the trim tabs to get on plane? Sometimes the tabs will take the water away from the prop and you won't be able to get on plane.

Mark

Hi i did not use the tabs, and this week i will try 4-blade 17,5x28,cleavers and drain out 200gal, of fuel before i try it again.

Now i have put the 2 inch spacers and 4-blade 17,5x28 cleavers,the engines are spinning 3000 rpm WOT 47mph still have difficulties to plane,think the max rpm is 6000-7000 rpm on these engines, gear ratio 1,5:1 ,what to do ????.

You will definately need to try smaller pitch props. It will get the engine into its power band. I would try something in the 22-24 pitch range.

Why do you think they are 6000-7000rpm motors???? Do you have dyno sheets???? With 620hp, 5500rpms is more likely.

Don´t have the dyno sheets, i pulled out the engines and i am going to supercharge them and do the dynos.

How did he get that from the Navy without holes being poked into it?

It had maybe 15-20 holes in the hull made by a forklift.

What gear ratios are in the outdrives? You should be able to spin more rpms than that with those props.

The gear ratio on my speedmaster#4 is 1,5:1

HP to weight ratio in theat boat should put you in the 70 mph range with 24 pitch propellers turning 5400.

Check your recomended engine rpm ratings. I think you may be a little high in 6-7000.

Because the propellers are now hooking up well enought to keep the slip % below 15, something must be going on with the engines to only allow 3000 rpm.