Prop slipping and not getting on plane
 

Not sure if this is a rigging or prop question but here goes.

I converted my 35 ft monohull from twin V8 2 stroke Evinrudes to two 496’s.

Took the boat out for her first run and it took a long time to get on the plane as the props were slipping (cavitating?). She cruised beautifully at about 50mph (guess as no speedo) and then the props would slip over that and hit the redline with hardly any increase in speed.

As it was the first run ever I used the props that had originally come with the boat, some 4 blade 24p cleavers. As the exhaust flows over the blades I thought that was the problem so put on some Mach Stern Drivers 4 blade 26p that I had purchased years ago of a fellow Offshoreonly member.

Could not get on the plane with the engines bouncing on the rev limiters. Lots of what appeared to be steam from the props. Couldn’t believe they were worse even though the exhaust went through the hub now instead of over the blades.

The boat is not stepped and there is no pad. She weighed just over 3000Kg dry with the V8 Evinrudes and pods so now probably 3500kg or 7700 lbs. Bravo diesel drives 1.36 ratio, the prop shafts are about 5inches below the hull.

Just a little edit.

The Mach Stern Drivers are 14 x 27p, not 26’s as previously thought.

So any suggestions at all on what’s wrong?

some of the possibilities are:

Too much pitch;

Not enough diameter;

Not enough cup;

Drive height too high;

Engines not producing enough power;

Need trim tabs;

Any other suggestions?

Summer is approaching here and would love to get the boat going.

Here are a bunch of options:

See if you can get your hands on a set of Bravo 1 26 pitch 4 blade. I think they will bite more than the Mach and will probably get more bow lift. Even though you are at 5 inches that is very conservative on an older hull. My V hull is at 4.5 inches with extension boxes and its tough to get on plane. The Bravo 1 is probably your "go to" universal prop for a mid age deep V.

You could also try spinning the props in just to see what happens. Right now you are spinning out. Reverse the propellers from side to side and swap the cables. The two lifting strakes exit into the prop area arent helping you so this might help.

You could try a set of 5 blade Maximus at about a 22 pitch.... They will work but might be a little rough on the drives.

If none of these work, buy a 1 inch drive spacers and it will space your lower DOWN 1 more inch. Then you start all over again to find the best prop.

Keep trying, you are close!

Your drives are not deep enough, the vent plate should be right at the bottom of the hull. IMO your way high

Drives do appear to be mounted high. Spacers will quickly verify if that is the case.

I gotta agree. The drives appear too high. Before you add spacers I would try some Bravo 1 props and some Rev 4 propellers. They have a lot of blade area and might hold the water a little better.

I would also concur with trying a spacer after experimenting with props to lower the drive.

Thanks for the replies.

Being in Sydney Australia parts are hard to come by. Just about everything has to be sourced overseas.

I guess trying the props the other way is very doable. Can I just leave the cables as they are and put the gear shift lever in the opposite direction?

No idea where I can borrow a set of 26 Bravos as the prop shops I have rang in Sydney go to about 22. Also no idea where to find spacers.

I guess today’s job is to work the phones and see if anyone has 26 Bravos and/or some spacers.

Thank you very much for giving me a direction.

Yes, you can swap props side to side and then use reverse as forward and forward as reverse leaving the cables as is.

LOL, yeah, it makes docking VERY interesting!

Off topic but........
I went out in that boat with Tony Lowe many years ago. Launched at Patterson River (Melbourne) on a snotty day. The V-8's were damn noisy and not very fast but she rode beautifully in the slop. Overall, I was pretty impressed.

I've often wondered what became of the boat because I'm an outboard fan. I'm also curious "Why the inboard conversion rather than just update to to more efficient outboards?" I see quite a few people converting boats to outboards but not the other way round....

Cheers mate!
RR

P.S. The OP has glossed over the fact that this is an awesome boat. As I understand, it was built by Tony Lowe and his race team (Trident engineering) and is a Cig 35' hull. I think it was built under licence from Cigarette but he could not use the name.

In addition to the drive height you would be better off with 1.5 ratio. In the mean time you should really try some larger diameter props.

Do you have an idea what the top speed will/should be? Unless you are anticipating 80 mph 26/27 is way too much pitch. If you run mid 60's the 22's should be just about right, For sure try some Revolutions if you can get your hands on them.

Tony was the importer of Cigarette at the time Don passed away. He couldn’t get stock as the factory was a mess and he decided to built a new class 2 race boat hence it was built pretty light and set up for a single big block.

The story I have been told by Simon that was working with Tony was:

During construction a ski boat customer of his wanted the boat and kept offering more money till he sold it to him. The ski boat customer also saw the V8 Evinrudes in the shop and wanted them so it was completed as a cruiser and sold.

When I finally had enough of 8 hours maximum without melting a piston I decided to repower. 12 years ago outboards were nowhere near as powerful, or as quiet, or as fuel efficient. That’s how long it has taken to get to this stage.

.

As for top speed, not sure. It did just over 60mph with twin 275hp outboards. Guessing maybe 70mph with twin 420hp as per the Bam speed calculator. Then using the prop slip calculator set about 10% I need 24’s.

Unfortunately I just ordered and paid for a pair of Bravo 26’s

Have you calculated slip at cruise or wot? 10% would be low slip if you’re having problems getting putt of the hole, so with higher slip 26s may be closer to what you need. Bravos are actually shorter than what they’re advertised as well, a 26 should be put in as 25.3 into a slip calculator.

The slip I am guessing on where it might be when dialled in. It is all theory so far, spent way too much time with google.

It is reassuring to know the Bravos will be a bit less pitch than advertised.

The Bravo 26’s should arrive early next week and the Chopper 24’s are at a local prop shop getting a once over. The prop shop showed me how the chopper props rock when placed facedown on a flat surface so the blades are not even which could be some of my problem. Looking at the Machs they have much smaller blade area and less diameter so with my relatively high x-dimension that makes sense that they wouldn’t push the boat.

All I have to do now is get some time off work to test the new props mid week when the waterways and boat ramps are not too busy.

I think 10% is a wet dream with that hull. Big heavy straight bottoms aren't very efficient. 10% is something you see in stepped hulls that have been played with to get props right. Avg slip is usually in the 15-18% range. Don't try to estimate slip you have to get on plane test wot and find your exact slip and then go from there.

My Bravo 26 props arrived today, except that they are Lexor 26’s by Solas. Looks like we mis communicated.

Guess I better send them back.

2021 what was the end of the story ??

My lounge room had about 5 sets of props as I tried quite a few.

In the end the original 24 props that were on the V8 Evinrudes were given some love by a local prop shop and they have been the best props so far.

Takes a little while to get on the plane, probably need to go deeper with the drives but gets to just over 65mph and close to the rev limit.

It is incredibly quiet, smooth, effortless once on the plane and super economical. On par with my twin diesel 30ft RIB.

The centre of gravity has moved forward a bit with the sterndrive configuration making it even better when we head into the ocean.

As far as I am concerned it is a success and would like to thank everyone for their input.

Unfortunately I don't have much time to play with it as I now spend most of my time on our Sunseeker.

I think the engines are up to about 30 hours from new.

I guess I am just getting old.