Prop Advice For 16 OB With Promax 225

[Greg Guimond]

Any opinions on possible prop selection would be appreciated. The 1974 16 Ski Sporter OB has a fixed 8" setback bracket and the 1999 Promax 225 has a Sportmaster lower unit. Thanks in advance.

[ChromeGorilla]

Holy crap! I recommend appropraite sized DEPENDS! :eek!:

Although I have no idea about the correct prop for this set up.... would love to see some pics!:yes:

[Greg Guimond]

This 1974 16 OB hull has a small modified pad that removes the dreaded coke bottle bottom. That said, prop selection is going to be a challenge with that much power so I hope the folks on this board can offer suggestions based on experience.

[Lenny]

1999 Promax 225 has a Sportmaster lower unit.

I got a little "depends action" on this too.

You will definately be fast if that is your goal. Upright is a better one...

;)

[tigweld]

I have a similar hull, but still have the round keel:wink:.

It is a 1971 17' Formula, same bottom as the Formula Jr.. When I first set up the hull after restoration I had a 200 Xri merc. The boat ran best with a 26 big ear choper . I now have a Jay Smith cut 2.5 and run a 27srx, or a merc ET. With that hull you will need to surface the prop, and run an over hub wheel.

[Greg Guimond]

Thank you "tigweld" for the prop advice. I'm not sure what advantage the small pad that this hull has will ultimately provide at top speed. My immediate interest is to prop to eliminate chine walk instead of attempting to attain the last 5 MPH. I am new to this powerplant so RESPECT is the first order of business and I appreciate any and all technical input with respect to prop selection that board members might provide.

[RBT]

try a 24 or 26 pitch small hub trophy, or Mercs tempests. But my bet is you will like the trophy better than all the rest for all around running.

[Greg Guimond]

Thanks "RBT" for the prop info. The hull will go in this weekend for a first run with a down and out 23 and then start to adjust from there based on board member input. Thanks to those who provided technical insight.

[Barry Eller]

I used to run a 16' Tahiti with a 200 Black Max. The best performing prop on it was Mercurys 28" Quicksilver Chopper. With this prop, I had much less "Prop torque" and much less chine walking. Just be careful on hole shots, the exhaust flows over the prop, not through the prop. You could get the RPM's up VERY quick. Once you learned it, you could get into the powerband RPM'S instantly, and fly.

[BUIZILLA]

diameter is going to be your enemy here..... thats what toss's the small hull around, so with that said, *I* would stay under 14" for sure, probably 13 3/4ish, and maybe start with a 26 pitch.... I like the chopper idea, the Action 17's liked those and they could hang the bow out at will pretty stable, I know the 225 ProMax in my eyesight has the rev limiter set at 7050-7100 and it's been buzzing 6700-6800 pretty happily so far :).. it ran cough93sumthincough on a 20' bass boat.. a GOOD 225 ProMax will dyno about 245-248 hp @ 6700ish :eek!::eek!: fortunately it only weighs 392#, BUT with your setback bracket the CG is moved back to an even more interesting V1 rotational point.. :shades:

[Barry Eller]

diameter is going to be your enemy here..... thats what toss's the small hull around, so with that said, *I* would stay under 14" for sure, probably 13 3/4ish, and maybe start with a 26 pitch.... I like the chopper idea, the Action 17's liked those and they could hang the bow out at will pretty stable, I know the 225 ProMax in my eyesight has the rev limiter set at 7050-7100 and it's been buzzing 6700-6800 pretty happily so far :).. it ran cough93sumthincough on a 20' bass boat.. a GOOD 225 ProMax will dyno about 245-248 hp @ 6700ish :eek!::eek!: fortunately it only weighs 392#, BUT with your setback bracket the CG is moved back to an even more interesting V1 rotational point.. :shades:
Buizilla is correct on the small diameter. My chopper was 14". I had a 9" set-back transom jack on the Tahiti. I ran a 28" prop as my lower unit ratio was 1.82:1. Drag racing gears! The Tahiti was clocked by a law enforcement buddy of mine with his patrol car radar at 84 mph. It was a handful to drive, with a round keel bottom and a hook very similar to the Donzi 16. I hope you are using a foot throttle and wearing a GOOD PFD, and an "ATTACHED" KILL SWITCH LANYARD.
P.S. I think my ability as a child to ride a unicycle helped me drive that boat....

[Greg Guimond]

Thank you "Buizilla" and Barry for the prop advice. This 1999 EFI Promax 225 has a Rapair ECU upgrade and 100 freshwater hours. I have some lines out now to find props that are <14" in diameter based on your respective comments. The steering on the boat is standard Hynautic and the old gas tank has been replaced with a new 32 Gallon that sits further back in the keel. I'm sure that the maiden run will be revealing and again thanks for the technical info as I am new to all this.

[BigGrizzly]

When you get a base line get back to us I really don't go with just the small diameter I go for a 4 blade maybe an over the hub lightning OTR . The old Chopper prop has been replaced in all our tests. with several brands of motors. Remember your in uncharted water with the pad and the setback on that particular hull. There is a balance factor we need to check out. Maybe the old stuff will work, maybe not. Like my Grand Dad said "Theory is good but honest testing is TRUTH". Good luck. If we can help feel free to call.

[WingWing]

you probably already know this but different props do different things. Basically, chopper style will give you bow lift, ET will be relatively neutral, and a cleaver will give you stern lift. What is your boat doing right now should be the first question to get an idea which way to go.

Normally I would suggest a chopper style like a mazco RE3 or something along the lines of a tempest or trophy like RBT mentioned (I think a lot of the heavier hulled SOB's run these). However, given your setback, you might want to look closer at the ET's (unless you need a little bow lift). mercury has a new 4 blade ET, if you do not need any bow lift and have the wallet for it.

good luck and post some pictures if you get a chance, especially of the pad. It would also be nice to hear back on your results if you don't mind, it sounds like a neat project you have going there.

[Greg Guimond]

Conditions were far from ideal this past weekend on LI Sound but with a 13 3/4 x 23 3 blade over hub no name prop I was only able to GPS 61MPH. RPM was never really any issue as I still had a lot to go. It felt like when I trimmed to where the rooster tail was excessive that the hull ran free but then began to become unruly almost immediately. I will say that the midrange punch on the Promax induced a big grin although I need more seat time and better conditions to get a better read on possible next steps. I will also attempt to post setup pictures when I get back to the boat next weekend. Thanks board members for your feedback.

[Greg Guimond]

Maiden voyage was with a 13 3/4 x 23 no name 3 blade and 61 was seen on GPS. To free up the hull, I had to trim excessively and then the hull wandered significantly. Chop was excessive so limited data back but there was plenty of RPM left with the Promax. This is the initial baseline and thanks to board members for the opinions based on experience. I will attempt to get pictures this weekend.

[Greg Guimond]

Well conditions were not good last weekend but with a 13 3/4 x 23 no name 3 blade I GPS'd 61mph. The thing that was most prevalent was how much trim it took to free the hull up. The hydraulic steering seems very tight and the midrange punch was pretty amazing. I need to gain more seat time but thanks to board members for comments and I will attempt to post pictures of the transom after this weekend.

[tigweld]

have you tried it yet? Any updates?

[Greg Guimond]

Well with more seat time I still could only GPS 61 before the hull becomes a handful with the 3 Blade 13 3/4 x 23 prop. My thought is to try a 4 blade next and see if that allows for less trim on the Promax 225. The midrange punch is certainly an eye opener with this motor.

[tigweld]

What is your engine hight(propshaft centerline to pad), and what steering do you have? I dont think a 4 blade will make a big difrence. It will all be in setup, and learing to drive a pad V:yes:

What was the rpm at 61?

FYI you will need a bigger Wheel:wink:

[Greg Guimond]

Here is an attempt to post a picture. I have no experience with this so I'm not sure it will work. The photo was taken by a freind so it is not the best.

[Greg Guimond]

I had ample running time today and still can not get past 62 on GPS. The RPM's are 5,000 at 50MPH and 6,000 at 60MPH. I wonder if the Sportmaster LU in a mismatch for this hull. The motor should be capable of spinning at 7,000 RPM so I'm not sure what to make of it. The boat just gets tipsy at sixty. Any feedback would be welcomed.

[Lenny]

Maybe a Jack Plate to release/increase the drive from deep/shallow water and tabs for stability ???

Jamie saw about 80 GPS with a small block on the same boat but an I/O and more ponies, BUT, he was 3" higher than a stock location prop centerline when running... (ALPHA SS)

Maybe that is the key. I am sure you will hear from him.

Maybe Mr. Haas can shed some light ... ;)

Where did that PAD come from? Looks JUST like mine on my "18" and all that OB support stuff on the transom? Is that STOCK?

Maybe you are NOT deep enough and with the wrong prop, but since you can NOT spin that 225 ProMax to redline I am totally confused. Seems, with the lower I see in the pic you should be on the limiter at LEAST !!! ???? :confused:

http://www.donzi.net/forums/attachment.php?attachmentid=19017&d=1141329003

[BUIZILLA]

Greg, what is the purpose of that horizontal fin attached to the cav plate?

[BERTRAM BOY]

This 1974 16 OB hull has a small modified pad that removes the dreaded coke bottle bottom. That said, prop selection is going to be a challenge with that much power so I hope the folks on this board can offer suggestions based on experience.


Not only did someone add a pad, but they extended the inner strakes as well?

Do you have a hull number?

[BigGrizzly]

I read the post again and I keep hearing the old fallacy about small diameter handling better. that was true of the old style chopper prop, which almost no one uses anymore. The old 16 we put a pad on also got loose about 60. In your case we have several problems. It seams that the strakes and pad added were like the old Scarab.design. Problem may be relaed to the angle of the pad in relation to the strakes. We also have a prop issue in the fact max rpms are not achieved. After working with some boat hulls like the Allison the engine hight his a big player. A 4 blade may add stability especially with the engine hight. I see some problem with not enough blade surface in the water to help stabilize the boat.

[tiger lily]

Greg, what is the purpose of that horizontal fin attached to the cav plate?

ive seen those before on dinghy outboards and pontoon boats, something to help them plane off faster, something that prolly should be removed

[Ed Donnelly]

Buizilla; I lengthened and widened my cav plate on my E lower, to prevent prop blowout coming out of the hole on my twin turboed 16'.....Ed

[Greg Guimond]

"BUIZILLA" the larger cavitation plate came with the 1999 Promax 225 when I bought it. I don't think that it is terribly helpful on hull re-entry although I have not yet tried the hull with it removed so can't comment on how the hole shot might be effected with it off.

"BERTRAM BOY" the HIN is DMR161060174-8.

I will attempt to post another photo of the transom without motor based on the comments of "BigGrizzly" and as always appreciate the feedback.

[tigweld]

Boat set up and learning to counter steer against the chine walk are what you need to focus on. The boat is now scrubbing of speed with the chine walking. Get the boat flying straight, and it will be Fast. To make that boat fly, with a surface piercing outboard you will need a number of things, some of which you probably have covered.

1- Good quality steering. Dual cable, Sea Star PRO, or external wing plate style. You must have all the slack adjusted out!

2- Solid engine mounts. Good steering doesn't help, with loose factory mounts.(with the pro max, you should already have solid mounts)

3- Proper boat set-up. This means engine hight, engine set-back, and weight balance of the hull. The engine hight on a surfaced outboard is usually the prop shaft center line even with the bottom of the pad + or - .5". If you are dragging the gear case, it will chine walk, and be almost imposable to drive. The engine set-back looks close for a start. 6" of setback is common on a 16' boat. how is the weight balance? Dose the boat porpoise, and get flighty a top speed, or do you have to trim the junk out of it for the bow to fly?

4- Learn to counter steer, and drive! We all have been driving boats for 20+ years, but this is more like a cross between flying an airplane, and riding a bike. I wont get into it here, but the easy way to learn, and see if there is anything wrong with your setup, is to have an EXPERIENCED pad V guy go for a ride. let him watch you drive, and watch him from the passenger seat. If you want a good book, call Allison boats and order a copy of "The little Book". No pad V will drive its self above a given speed, you drive it...


PS.
I wouldn't get rid of the sport master yet. It is not causing your handling problems. For speed it could scrub of a little, under 80mph, but I bet your boat will approach that when you are done. Besides, you need the low water pick up, and the sporty is strong and looks bad a$$.

Marc

[Greg Guimond]

"tigweld" thanks for the comments. The boat does have hydraulic Seastar steering although not the more recent Seastar Pro. The helm has been balanced so that there is no play in the steering ram. The Promax 225 comes standard with solid mounts so that is no longer a concern and the Promax is lighter than the JohnRude 225 that was on the hull prior to my purchase so balance should be as good or better.

The set up is the largest obstacle and I think that perhaps the next step is to remove the larger cavitation plate and see what happens along with more seat time. I will try and post another photo or two.

[Greg Guimond]

Several changes were made this weekend which allowed 70 MPH to be broken. The removal of the cavitation plate and a prop swap to a 13 3/4 x 25 4 Blade Trophy. GPS is now 71 MPH but we are right back to the chine walk problem at that speed. All suggestions are welcome and my thought is perhaps the Promax needs to be raised up one notch on the setback bracket.

[BUIZILLA]

diameter is going to be your enemy here..... thats what toss's the small hull around, so with that said, *I* would stay under 14" for sure, probably 13 3/4ish, and maybe start with a 26 pitch.... for once i'm right :wink:

[Greg Guimond]

"Buizilla" there is no doubt you were correct as we saw 69 MPH with the 3 blade prop once the enlarged cavitation plate was removed. I call it a cavitation plate as I am not aware of the proper name. We then put the 4 blade Trophy prop on and saw 71 MPH but it was much smoother getting to that speed than with the 3 blade prop. Many of you have looked at the photo of the Speedmaster lower unit in relation to the pad and the consensus seems to now be raise the motor one notch to see what the results are so that will be next. The Promax 225 still has leftover RPM and associated power so we shall see where things go. I have attempted to attach a picture of the Trophy prop.

I can say that the driver gets zero warning before the chine walk hits.

[BUIZILLA]

prop it to 6600-6700...........

[Greg Guimond]

Thanks to the board members for the input so far. The 1999 Promax 225 has been upgraded with SVS and Rapair ECU which brings the top RPM's up to 7500. The small shaft Sportmaster Lower Unit will allow us to now focus on raising the engine height to see if we can eliminate the nasty chine walk that is occuring at 70MPH. For the moment the 4 Blade 25 will remain the test prop and as always all advice is welcome as we continue on with lanyard attached!

[tigweld]

The sportmaster is good for 100+. They are usuly a little slower on a sub 80mph boat.

[Greg Guimond]

The Promax 225 has now been raised up two of the three available bolt holes. The center of the Sportmaster's small shaft is now even with the bottom of the 16 OB's pad. With some luck, the weather will allow for testing this weekend. We also have a "Big Ear Chopper" waiting in the background.
As always thanks for the messages and advice that board members have supplied.

[tigweld]

let us know how it runs.