Porpoising Seavette

[lost2a6]

Funny, Capt Chuck. I'm going to try 100lbs in the anchor locker to see what effect it has on it. The weight of the engine and drive is about 1200 lbs plus two batteries. Also the fuel tank is right in front of the engine. Plus everyone is sitting in the stern of the boat. When I slow down, the little rooster tail that follows up to the transom will actually splash over into the boat.
It doesn't have any hint of chine walk.

Frank, my cavitation plate is 2" above the keel which puts the center of the prop shaft 7" below the keel. According to the guys over at offshoreonly.com this is too deep, however I don't know because at cruise I can't quite get the drive at neutral position and the prop starts to cavitate. I feel that it still has a few more MPH in it if I can get it all dialed in.

[Fr. Frank]

What's your prop? Are you running the original TRS drive?

[lost2a6]

Yes it is the TRS drive. So far I’ve tried a Lazer II, Ballistic, Mirage, and Revolution 4. The Mirage is the fastest however overall I believe that I like the Rev 4 the best so far. I want to try a semi-cleaver and a Bravo prop.

[McGillicuddy]

Everything you mentioned but the Rev 4 is a bow lifter, I'd hold out for Fr Franks opinion, but I think you skip over the the Bravo series and the semi-cleaver and jump into the cleaver category. Get the rear end up in the air, keep the pointy end down, "bend the throttle and let 'er eat" as has been oft repeated here. I have never run that fast so this is just theoretical not my experience...Heck I don't even like to drive that fast

[lost2a6]

I feel that a cleaver might suffer at cruise due to not having enough blade surface area, however I'll try anything. At cruise I'm running 3500 RPM, 40 MPH and getting 2.4 MPG so I don't want my cruise to suffer just to get more MPH.

[Fr. Frank]

Everything you mentioned but the Rev 4 is a bow lifter, I'd hold out for Fr Franks opinion, but I think you skip over the the Bravo series and the semi-cleaver and jump into the cleaver category. Get the rear end up in the air, keep the pointy end down, "bend the throttle and let 'er eat" as has been oft repeated here. I have never run that fast so this is just theoretical not my experience...Heck I don't even like to drive that fast

Actually, the Mirage and the semi-Cleaver and Cleaver props are all stern lifters. The others mentioned are bow lifters. I use the old-time (early 1970's) definition of a semi-cleaver being a through-hub exhaust prop with true straight back cleaver blades. A full cleaver is over-the-hub exhaust.

Here is a cleaver.

Here is a semi-cleaver:



and here is an almost-semi-cleaver from PowerTech, the OFX3:


I truly don't know which will give you the best cruise performance, but I lean toward the almost semi-cleaver/semi-cleaver (or thru-hub cleaver). This may cost you a little top-end performance, but will cruise at 75% power better than a full cleaver.

75% power is defined in terms of hp produced, not in percentage of max rpms. For example, for my 90 Optimax, 75% power is 4600 rpms, which is actually 82% of max 5600 rpms. For my old t/260hp 26' Nova II, 75% power was 4400 rpms, 91% of the max 4800.

It's important to remember that any cleaver-type propeller blade is at its most efficient when it is run as a surface-piercing propeller. The amount of blade surfaced for best performance is a matter of constant trial and error.

Cleavers also ALWAYS cost you out of the hole performance, because of the cavitation induced under acceleration.

For the record, I ran thru-hub "semi-cleaver" props on my 26' Nova II (and both RH drives) after trying a lot of props. That was partly because I ran offshore a lot, and cleaver blades don't shock the drive-train as hard on re-entry after launching off the top of a swell.

[McGillicuddy]

Thanks for clearing that up Fr. Frank, I figured if I spouted off I'd learn something yet again...Didn't think economy at cruise was an issue to a dude set on running over 60mph On the other hand you can do more often if there's more gas left to burn

[Fr. Frank]

...Didn't think economy at cruise was an issue to a dude set on running over 60mph On the other hand you can do more often if there's more gas left to burn

You have a point, here. But SeaCrafts CAN be made to run over 60, and I have seen a 20' SeaCraft with a slightly modified hull approach (but not quite reach) 70 mph.

I didn't do the hull work, but I did tune the rig on a 20' SF with a 240 hp. Merc Bridgeport on a hydraulic jackplate. We ended up running a custom prop from 2nd Effort in Lk. Hamilton, FL. The boat was REALLY, REALLY Squirrelly from about 55 to 62-63, and then settled down until reaching 68, which was WOT. Without trim tabs and the hull mod, there is no way it could have been pushed past 62.

The hull mod was done by Rybovich/Spencer in WPB, and included a flat planing pad on the rearmost 5'-6' of the center V, much like the early Steve Stepp 22' Velocity hulls. With a similar modification, I believe a 20' or 23' can easily be pushed past 70 mph with modern outboards.

I think your hull, Steve, can be pushed to near 80 mph with a similar mod, and a change in drive and engine, ..say to Mercruiser EU662-SCI.

[gss036]

I just cannot imagine running a small boat that fast. You sure could not do that in our waters here, there is just way too junk in the waters. Right now, it would be dodging the tribal crab pots and kelp. Me, I am just a simple boaters who likes to cruise at 25-27 MPH.

[peterb]

Over 60? Yikes!!!!

I am more of slow and steady kind of guy. I try to keep it to around 55 or so

I got a pair of rev 4's last falland have yet to try them out. I am looking for some more stern lift.

Peter