TOPIC: Finally Found Flat Water.......

took the mantaray out today with my bro-in-law. lake hopatcong was flat as can be, a rare occurence. an extra 60 lbs in the front made no difference in the porpoising. i raised the motor up to the 3 hole and cavitation was terrible. it did this when the 9 gallon tank was full right down to a 1/2 gallon left. so weight difference in the rear didn't change anything. the porpoising stops at 28mph..4000rpms. spent 3 hrs on the lake and when we got back to the ramp the prop was tangled in weeds and fishing line. there were a few people at the ramp and they said the crap on the prop hurt the performance. i could only get it up to 4800rpm. in june i was turning 5400rpm, so i guess the weeds were the difference. i'm buying a single trim tab from joe poole, he thinks will help a lot in reducing the porpoising and might eliminate it entirely. for $35 bucks its worth a shot. it will be mounted at the top of the tunnel in front of the cav plate. frank a. is going to help me mount it. have to drill 4 holes in the transom. i know the correct way to solve this is to raise the motor higher up on the transom but this ain't gunna happen. hopefully the trim tab will work if not i'll try the thingies that mount on the ventilation plate. also gunna try the 5 hole as someone said that will bring the nose down. i will be going back up oct.2nd for an aomci wet meet, will try the 5 hole then. ron

It sounds to me that if you are cavitating and porposing, that you are trimed out too far. You said #3 hole, is the trim hole or mount hole? We all know were the cav. plate is "supposed" to be, but if you are trimed out too far, you are tilting the cav. plate and prop up causing cavation and porposing. Before I drilled any holes, I would tuck that prop up tight to the transom, if you haven't already tried that, and work out from there.
I had a bass boat( a long time ago) that cavitated bad with a steep pitch prop, the engine spun it real easy, too easy. I finally got a prop with less pitch but more blade area, end of cav. problem.
old darb

yes darb,trimmed out to far.will tuck it in more next time out. but i'll probably still get water over the transom into the splash well, because the leg sits too low. ron

Mine had a 20" transom and had a short shaft. The motor was posissioned where F_P sugested in my motor thread. The boat also had a metal tab/flap over the tunnel, see pic below, looks like it was installed the same time as the transom plate. Seems like cavitation would be a problem that high unless runnin' a chopper or cleaver style prop??? I think the chopper lifts the bow & cleaver lowers the bow. I need to learn more bout these. There are many options to stop the porposing. I've only had the problem on one boat and a Sting Ray, wing style, fixed it. I do know it's easier to adress the problem from the stern and use the leverage of the boats length to hold the bow down.

Props like undisturbered water. That is one of the reasons Volvo's new pods put the prop in front of the lower unit.
Cat hulls mix the wake between the two hulls. The closer the hulls, at the transom, the more disturbered water. AIR!
The picture shows a 2 blade prop. Looks like a lot of pitch(?). 2 blade props are usually speed props. Great for speed, but they don't like to get out of the hole.
If, when you punch it, from stand still, it cavitates, the prop is spining too easy, making a votex and sucking AIR down (bass boat).
Try a 3 blade prop with more blade area, even with cups.
If there is a prop repair shop in your area, check with them for pitch and diameter.
old darb

its a 17 pitch prop. i'm taking it off after the wet meet a week from tomorroww and giving it to the marina mechanic to be cupped. ron

Good for you Ron, that would be my suggestion. A cupped ss 3 blade prop is what got my nose out of the water and the boat finally performing as it was meant. I think ss does better than aluminum because it doesn't flex, but I am no expert!

What rpms are you getting now? Can you open it up to full throttle once in a while just to see?

Good luck!

Frank

Spending time with my brother-in-law.....Priceless! Ha!Ha!

Sorry, I just couldn't resist commenting....

PC1000 wrote:

Good for you Ron, that would be my suggestion. A cupped ss 3 blade prop is what got my nose out of the water and the boat finally performing as it was meant. I think ss does better than aluminum because it doesn't flex, but I am no expert!

What rpms are you getting now? Can you open it up to full throttle once in a while just to see?

Good luck!

Frank

i got it up to 4800 rpm but there was weeds and fishing line tangled on the prop. didn't take it back out after we untangled it. will try it again oct.2 at an aomci wet meet. i'm cupping my 2 blade bronze 17 pitch. marina mechanic suggested i drop the motor 1 more hole. he said it might stop the porpoising. the first 2 years i had the boat i think that was the position it was in because it never porpoised. should have the single trim tab in a few days. ron

Okay Ron, sounds good. I think 5200 is the magic # for you too isn't it?

You coming up sometime or what?

Frank

5200 or 5400 i forget now. how's oct.9th for you?

October 9 is okay Ron, got it on the calendar. Cool.

Frank

Looking at your picture your motor looks to be 4 to 5" to low, I see you said
" raising the motor isn't going to happen " just wondering why ? is that a 20" mid ? if so maybe a 15" mid is the way to go. You won't believe the difference raising that lower end will do for your performance !!

its a short shaft. i can't find anyone to raise it with the guarantee that it will solve my problems.
1.porpoising
2.water over the transom into the splashwell at speed.
3.cavitation

1) Trim. 2) The smaller plate above the cav plate is contributing to the spray, as does the water pump bulge, another reason to recommend raising the motor. 3) Cupping the prop will help, but trapping air is what a tunnel hull does. The correct prop will minimize, but not eliminate cavitation.

hmmm guarantee, I'd stop short of that

But I would say I'm sure your spray problem is the exposed wider area of your mid section as mentioned by Kerry above.

Tunnels and or stv's will porpoise at a certain speed until you get through it and start to " fly " I think your not able to get past it because of the motor being low which scrubs speed. Maybe fabricate a U bracket to fit over your transom which would allow you raise the motor might be an inexpensive way to solve your problem ? Or of course a jack plate would work but might not look the greatest on a beauty boat like yours.

You mentioned cupping your prop, what is the recommened top end rpm of your motor ? you don't want to put to much of a load on the motor by over propping.

Dan

i think top end is 5200 rpms and i've always attained that without weeds and such on the prop. again i would only make the modifications if the person doing it guaranteed his work. if i can't get a guarantee it will not be done and i'll live with what i got. i mostly troll the shorelines anyway lookin at houses. lol ron

I'm very interested in this as I may be in the same boat. My prop is in the same location as yours. Did you get the tab yet? From what I've found out I should have a short shaft with the 20" transon, and you should have a hydro/X-short shaft with the 16" transom. Either way a cleaver prop would be a must to run in the dirty (aireated) water. From what I've read a cheaver prop on the motors we are running still may be the answer. It would allow the prop to grab in the dirty water when the motor is tilted up a peg or 2. As far as diameter & pitch I'm still working on how to figger out what would be best to start the trail & error.

Some good readin'
continuouswave.com/whaler/reference/prop1.html
continuouswave.com/whaler/reference/prop2.html
continuouswave.com/whaler/reference/prop3.html

en.wikipedia.org/wiki/Chopper_(propeller)
en.wikipedia.org/wiki/Cleaver_(propeller)