A couple WOT passes tonight

[Firehawk853]

Took the Donzi out to try out a 21 pitch 3 blade prop tonight. Made 56mph on the gps. Compared to the 17 pitch / 5 blade, this prop feels sluggish, not as responsive, opposite what i would have expected. However, it rides MUCH nicer, no bouncing! I just rebuilt the engine, so I'm unsure exactly what to expect.

I think that the 21 may be a bit much for this boat, especially if I have a few people in it and a full tank. I tried 3 people tonight and it had a tough(er) time than 2. I'm thinking maybe a 20 pitch is the ticket.

What speed should I expect to hit? I was hoping for 65 or better seeing as the old owner claimed it did 62....

[MOP]

Post more info, boat, engine, WOT RPM! Randy "Big Griz" will come up with a good starting point. A jump from 56 to 65 is quite a reach but may be doable with the right combo.

[onesubdrvr]

OK,

I had a 21' ragazza with a pretty well built 355 (I think, 350, .030 over), with Vortec heads, a bumped up cam, and a better than stock exhaust (not CMI's or anything, but better than stock). We figured I was putting about 365 to the crank. Not sure what your HP is, but this should give you some idea.

I was able to get 61mph GPS, and the person I sold the boat to claims he's had it to 63gps.

So, depending on how much work was done to your engine, 56 might be pretty close. I was using a Turbo I 22p. Yes, sluggish out of the hole, but better top end.

If memory serves me correctly, when up at these speeds, you're looking at 10-15hp per MPH, so taking the stock 350 @ 270hp, and mine at 365, the stock motor probably would have run about 52. So, 56 might be close with a couple of more for playing with props. All depends on what kind of power you have.

Wayne

[Firehawk853]

We just rebuilt the engine, so I would guess in the 350-385 range because I didn't have it dyno'd. The engine is a .040 over stroked SBC, 9.3:1 compression, 200 World Iron heads, the cam is agressive but I can't recall the specs, Imco thru hull

I borrowed this prop to test the results
This run we used:
3 blade 21p
WOT RPM was ~4500-4600
56 mph gps

My only reference at this point is the prop it came with:
5 blade 17p
WOT RPM was ~5400-5500
This seemed more responsive but very hard to keep from bouncing, also bogs pretty hard turns.
Didn't have the gps.

with less pitch (say 19 or 20) I would be able to turn a few more R's, so in theory would make up a few mph top end?

[onesubdrvr]

We just rebuilt the engine, so I would guess in the 350-385 range because I didn't have it dyno'd. The engine is a .040 over stroked SBC, 9.3:1 compression, 200 World Iron heads, the cam is agressive but I can't recall the specs, Imco thru hull

I borrowed this prop to test the results
This run we used:
3 blade 21p
WOT RPM was ~4500-4600
56 mph gps

My only reference at this point is the prop it came with:
5 blade 17p
WOT RPM was ~5400-5500
This seemed more responsive but very hard to keep from bouncing, also bogs pretty hard turns.
Didn't have the gps.

with less pitch (say 19 or 20) I would be able to turn a few more R's, so in theory would make up a few mph top end?
Pitch is the number of inches forward the prop moves per revolution. So though you may turn more rpm's, each rpm doesn't move you as far.

It's all about prop efficiency, and lots of things can effect that, including type of drive, depth in the water, and lots of other things. You really have to find the best prop for your boat. And a 21 in one brand may work good, but go to a 22, and speed and handling may suffer. It's trial and error.

You may find another 21p that gains you a few mph, at the same RPM because the added efficiencies are greater than the one you've got.

Your boat weighs a little more than mine I think (I had the open bow version), but again, at 365hp, I turned my 22p Turbo prop at about 4800 rpm on my top speed runs (if memory serves me right), for my 61mph (gps). I always thought the boat had more in it, but didn't keep it long enough to find out. Maybe I had a few more ponies than we estimated(?)

Wayne

[MOP]

Spinning high RPM's in most cases does not give you your best speed, you need to be propped a little above your max torque at WOT. In my particular case a small block 22C, when I switched from the BH drive to the Bravo the only prop I had handy was a 3 blade 21" which spun 57-5800 way to high for my liking but gave me 62. I grabbed a 23" cleaver which brought the RPM down to mid 5's and got me to 65 and was the absolute worst handling prop ever, on the advice of Big Griz I bought a Turbo Ultima. The Turbo is a 4 blade 23" which I spin at 5100 and gives me 65.5 on a good day, I make about 370HP with a mild 383 stroker. Randy was spot on with his suggestion, the Turbo is exceptional in rough water and handles great. Now don't run out and by one, all boats react differently to different props. Try to make a local Donzi event and try several there is one out there that will please you.

Phil

[blackhawk]

The 17 will feel more responsive because of the lower pitch, perfectly normal.

As Phil said higher rpms doesn't always equal more mph. However, IF your motor makes it's power at higher rpm then raising your rpm with a smaller prop may give you more speed.

[BigGrizzly]

The 5 blade is too small and probably not as efficient. It would help to know which 3 blade 21 you used. Every brand has a different pitch progression which will greatly affect the outcome and speed.

[Firehawk853]

The props I am using are

17P High 5 (5 blade)
21P Mercury Mirage stainless

I read about similar boats able to get over 60mph. With this engine and the claims of the previous owner, I would have it to be no problem to spin a 21p. So much for expected results.

I pulled the plugs last night, seems like it running a bit on the lean side, but I doubt this will make up 10mph.

[onesubdrvr]

The props I am using are

17P High 5 (5 blade)
21P Mercury Mirage stainless

I read about similar boats able to get over 60mph. With this engine and the claims of the previous owner, I would have it to be no problem to spin a 21p. So much for expected results.

I pulled the plugs last night, seems like it running a bit on the lean side, but I doubt this will make up 10mph.
Don't get discouraged, maybe the power is there, maybe there is some other problems.

One thing I would do, is weigh the boat, is it possible that any foam under the deck has absorbed large amounts of water?

I always go back to checking timing. It's possible that your timing is off causing some lack of power.

Compression test all cylinders, though it's been rebuilt, maybe something is wrong

Check firing order, 2 could be off, and it still seem to run right

There are alot of possibilities, it seems with that power set up you should be closer to 60.

Where are you running? are you running at any altitude because you'll loose power there

Just some other things to look at / consider,.... of course it is possible he told you speedo speed was gps speed, that's been known to happen before. Heck if I went by speedo speed, mine would have been a 70mph boat.

Keep you chin up, and lets see if we can't get you some more speed.

Wayne

[Firehawk853]

thanks again!

You bring up a good point about weight, we called Donzi yesterday to try and get the old records, but when OMC went under in '94 they took all the old records, so i'm not sure what is 'should' weight. The woman did say that each of these hull's were hand made and weights could easily vary depending on the how much glass was laid down.

In any case I'm guessing it weights about 3500, at least thats whats on the title.

I've fab'd up a wideband O2 sensor tube that I can jam into the thru hull exhaust. I will get in past the water jack so I will get a good reading.... Bring the tuning kit and see what happens. Its pretty much speculation until I know its tuned right.

[glashole]

keep in mind old owners are generally optimistic when remembering exact speeds or trying to sell a boat

[J & J Speed Shop]

thanks again!

You bring up a good point about weight, we called Donzi yesterday to try and get the old records, but when OMC went under in '94 they took all the old records, so i'm not sure what is 'should' weight. The woman did say that each of these hull's were hand made and weights could easily vary depending on the how much glass was laid down.

In any case I'm guessing it weights about 3500, at least thats whats on the title.

I've fab'd up a wideband O2 sensor tube that I can jam into the thru hull exhaust. I will get in past the water jack so I will get a good reading.... Bring the tuning kit and see what happens. Its pretty much speculation until I know its tuned right.

time for speed holes or a 22zx :smash:

[blackhawk]

I always go back to checking timing. It's possible that your timing is off causing some lack of power.

My timing being off cost me 6mph! Boat idled fine and seemed to run fine but would "burble and pop" a bit off idle if you opened the throttle too fast.