Prop Advice

[PlumCrazy]

First off, forgive me for intruding into a Donzi forum. My boat is a 1988 Baja Sunsport 196. Basically, it's a low profile 19-1/2' open bow that runs with most of the hull out of the water. Because the hull and hardware are very similar to what you guys are running, I believe this is the best place to ask advice. To give you an idea of how this boat looks and runs, here's a video: http://media.putfile.com/1988-Baja-20-Runabout

The engine is a 400HP 383 SBC and the drive is an Alpha SS with a 1.50:1 gear ratio. My best speed to date is 74.6 @ 5000 RPM with a 25P Mirage+ labbed by Bblades. I'm hoping to eek out a little more top speed. Just from searching here, I've found that the Ballistic and Laser II may be props to try. Any other suggestions?

[Last Real Texan]

A slip ratio percentage of .055 ........pretty good I would say.............where do you go from there? it would seem you have the optimum set up already..............is that a GPS speed?

Bryan

[The Hedgehog]

A slip ratio percentage of .055 ........pretty good I would say.............where do you go from there? it would seem you have the optimum set up already..............is that a GPS speed?
Bryan

LMAO! That is pretty tight. I would like to see some #'s like that in the X-18! I am thinking that Griz will be able to help get me dialed in well but that is pretty good.

[Pismo]

A labbed MPlus is hard to beat.

[PlumCrazy]

A slip ratio percentage of .055 ........pretty good I would say.............where do you go from there? it would seem you have the optimum set up already..............is that a GPS speed?
Bryan

Yes, it's a GPS speed. I use a Garmin 76. While the slip is only 6%, I've learned in the past that low slip is not always fastest. In fact, I get a slip ratio of 4% with an non-labbed Mirage+ 25P. But, it's 1.6 MPH slower.

[oldandtired]

Do you know what rpm your peak hp is at. I don't know this, but I have a feeling that you have a few hundred RPM to go. If you do, then a lower pitch may yield a better top speed. Good numbers BTW.

Rene

[PlumCrazy]

No, unfortunately, I don't have a dyno sheet. The engine builder recommended keeping it around 5000 RPM. I was thinking about labbing and de-pitching my stock 25P to a 24P. That would tell me if more RPM would help. However, the real reason I came for advice, was to learn if another prop design would be better. It sounds like the Mirage+ is my best choice.

[The Hedgehog]

I hear that Baha's really like the Mirage Plus props. It sounds like you have it dialed in pretty well.

[Last Real Texan]

Yes, it's a GPS speed. I use a Garmin 76. While the slip is only 6%, I've learned in the past that low slip is not always fastest. In fact, I get a slip ratio of 4% with an non-labbed Mirage+ 25P. But, it's 1.6 MPH slower.
That is really great efficiency to say the least....Dont know of too many cats that even run that well....I have a bud with a 22 ft Dayton tunnel hull with a blown 502 ( 650 HP ) that runs just shy of 100 MPH @ 5200 spinning a 32 in ch Bravo I Actual # 's are 95.6@ 5200 w/ a 1.5 leg. Com es out to be around .08 slip........he tested everything out there....
I am not doubting your #'s but for a non step old school hull , All be it a light small boat, I am very impressed at what you have achieved....Have you verified your Tach?
Sounds to me that a more realistic RPM for a built small block would be 5300-5400 and at those #'s puts you slip in the 10-12 % range which I would think is more realistic.
Just my thoughts....I think your tach is reading slightly low.....
Bryan

[PlumCrazy]

Yes, the tach was verified with a hand-held inductive tach.

[BigGrizzly]

I believe your numbers. You have a flatter dead rise on that 19 then the Donzi. I don't think that you will get more than 2/3 more mph out of that boat with a prop change and keep it at 5,000rpms. Blades also knew you have a SS lower unit and did the prop for it. I am sure that if you match up your prop with a stock M-plus they don't even look alike. You are partially correct on the slip theory providing you don't torque and horse power to pull through the wind and water chop. Prop design has a lot to do with efficiency, speed and slip. However your boat has a frontal area issue,that limits your speed. Don Jolly has a similar boat with a well done 383(with a SS) and we call it the cushion boat boat. At about 73->75 the front cushions unsnap and lift and come right over the wind shield and hit you in the head, if you don't duck. If we gave him another 50 hp he wouldn't gan a 1mph. You sound like your boat is set up well and you know it. You are just fishing for more speed. The other props you mentioned, I feel, would be a wast unless you can return them at no cost. I am all for trying stuff. If you want to try another prop for speed the Precision Lightning or another type prop designed to lift the boat a tad and run the higher X. For handling and more speed(yes) go for external steering I know your lower unit is fishtailing. The biggest other problem is, there is very little TRUE prop information on that hull! With a 18 or 22 Donzi or a Triton or Ranger bass boat there is tons of information. Even then there are several props to try. Good luck on your quest, it won't be easy, but at least you will know the answers.

[Last Real Texan]

Yes, the tach was verified with a hand-held inductive tach.
You are to be congratulated on your set up then. If I were you I would have Brett @ Bblades take your mirage plus and tweak down to 24 -24.5 and give you another 200 RPMS and see what happens. With the revs you are at I believe you are leaving some speed on the table....500 on a well built small block is nothing for it. turn it 52-5300....and hang on!

Bryan

[PlumCrazy]

I believe your numbers. You have a flatter dead rise on that 19 then the Donzi. I don't think that you will get more than 2/3 more mph out of that boat with a prop change and keep it at 5,000rpms. Blades also knew you have a SS lower unit and did the prop for it. I am sure that if you match up your prop with a stock M-plus they don't even look alike. You are partially correct on the slip theory providing you don't torque and horse power to pull through the wind and water chop. Prop design has a lot to do with efficiency, speed and slip. However your boat has a frontal area issue,that limits your speed. Don Jolly has a similar boat with a well done 383(with a SS) and we call it the cushion boat boat. At about 73->75 the front cushions unsnap and lift and come right over the wind shield and hit you in the head, if you don't duck. If we gave him another 50 hp he wouldn't gan a 1mph. You sound like your boat is set up well and you know it. You are just fishing for more speed. The other props you mentioned, I feel, would be a wast unless you can return them at no cost. I am all for trying stuff. If you want to try another prop for speed the Precision Lightning or another type prop designed to lift the boat a tad and run the higher X. For handling and more speed(yes) go for external steering I know your lower unit is fishtailing. The biggest other problem is, there is very little TRUE prop information on that hull! With a 18 or 22 Donzi or a Triton or Ranger bass boat there is tons of information. Even then there are several props to try. Good luck on your quest, it won't be easy, but at least you will know the answers.

Luckily, my cushions are permanently attached. So, no problem there. I've already addressed the open bow issue with a snap-on cover. It gave me 1 MPH. My best speed of 74.6 was with the cover installed. I've got to get this thing over 75, though! :yes:

[PlumCrazy]

BigGrizzly, thanks for your comments on the props. I have no previous experience with them and was wondering if I was wasting my time. I do have a question. Do you mean the Turbo Lightning?

When I bought this boat 4 years ago, it had a built 550HP 355 solid roller SBC in it. It also already had the Alpha SS. Compression was 11.5:1 and I ran 110 octane. To top it off, it had a 125HP NOS Sniper nitrous kit installed. The boat ran 79.1 @ 6000 RPM and 84 flat @ 6500 RPM on nitrous, spinning either a Quicksilver Mirage 23P or a Spinelli 26P 3-blade. Even back then, it didn't have external steering and behaved very well. Although, it felt like you were riding the outdrive. The only problem was that I was always wrenching on it and the fuel/nitrous cost was insane. It finally ran lean on a cool day with the nitrous on and popped the #2 piston. Right then, I decided to tame it down. The nitrous is gone and I've gone to hydraulic lifters. I figured with only 400HP, I would only see 70MPH. But, the boat proved me wrong.

The 383 can make a lot more power. I think after this next season is over, I may look at installing better cylinder heads. I wanted to re-use the Pro Toplines from the old motor, but we found hairline cracks and the budget at the time only allowed for stock heads.

BTW, I still have the NOS Sniper kit, if anyone wants to give it a try!

[The Hedgehog]

You really have it dialed in well. It would be pretty hard to get much more than 1mph without adding some hp. I would think that your SBC setup would hit peak hp at 5200 RPM or up so you might be leaving some on the table. You do seem to know your boat well though.

If you want to see 80+. I am thinking that you will need a blower pending on how much you dropped the compression ratio.

[RedDog]

Luckily, my cushions are permanently attached. So, no problem there. I've already addressed the open bow issue with a snap-on cover. It gave me 1 MPH. My best speed of 74.6 was with the cover installed. I've got to get this thing over 75, though! :yes:

Try running down stream or down wind and calling it good

[BigGrizzly]

Yes I am referring to the Turbo lightning. Precision is Turbo, they nicked named it after the success of the TurboI. the problem is people keep thinking I aqm talking about the turbo all the time. Since Merc was forced to drop the lightning name there is no confusion there. First I will say you have more than 400 ponies, and you have done a good job setting it up. Usually people screw up doing boat engines than propping becomes a horrendous experience. The cover solves only part of the problem. A solid cover would be a tiny bet better. The windshield shape, even as good it looks good has some problems. I have other Ideas that could help, short of doing a new motor. To involved to do on line. I won't do it for you but will tell you how to do it. you are welcome to call in the evenings.

[PlumCrazy]

Yes I am referring to the Turbo lightning. Precision is Turbo, they nicked named it after the success of the TurboI. the problem is people keep thinking I aqm talking about the turbo all the time. Since Merc was forced to drop the lightning name there is no confusion there. First I will say you have more than 400 ponies, and you have done a good job setting it up. Usually people screw up doing boat engines than propping becomes a horrendous experience. The cover solves only part of the problem. A solid cover would be a tiny bet better. The windshield shape, even as good it looks good has some problems. I have other Ideas that could help, short of doing a new motor. To involved to do on line. I won't do it for you but will tell you how to do it. you are welcome to call in the evenings.

I wish I could take credit for the engine, but the local performance marine shop put it together for me. I never got a dyno sheet, but according to his prop dyno, it was about 400HP.