Prop help on 18

[HyperDonzi]

Riding with a local Donzi-er, beautiful boat, got into talking about props...

1983 Donzi 18
Alpha SS Drive (not sure on ratio? guess 1.5)
Power-406SBC 9 hours since rebuild
curent prop and RPM-23 Lazer @ 5400

Wants to switch to 4 blade, any suggestions? What size? Hydromotive? Looking for a bit more stability.

Thanks,
Tommy

[GeneD]

I would suggest a 24-26' Bravo or a similar Hydromotive.
I know the 406 is going to love the RPM's but me, and this is me...I would love to get the rev's down to 5K.
Your going to love the acceleration on a 4 blade.
Most of the racers have Bravo's, some are going to the Hydro 5 blade, and some still are using the Hydro Quad IV.
I can only think of a few dudes that are using 3 blade Mirage Plus', but they are low budget outlaw boats.

[Rootsy]

first off.. where does his 406 make peak HP... this is where he wants to prop the motor to... within +/- 200 rpm or so. secondly, if it makes peak HP over 5K this is ok as long as the motor is put together correctly with quality components... a stock lower end is easily capable of 5500 without much fear after which you want to have some upgrading going on in my opinion, plus balancing. If you are gonna spin her that fast for any period of time you better have adequate valvetrain and A GOOD oiling system also... After that you can tailor a prop. the MIRAGE IS FAST but loose... the quad IV is large diameter, has awesome bite but takes more hp to spin due to the diameter and the extra blade.. Basically the hydromotive is a 4 blade round nosed cleaver... it takes all that diameter to get the bite... therefore you have to drop an inch in pitch vs a 3 blade to get the same rpm and end up losing 1 - 2 mph on average. but it is much stable... ask mr allard bout the quad IV on his 18 vs the mirage...

i cannot comment on the bravo one prop since i have no experience with em...

cleavers are fast and very stable but really blow in the midrange and give higher slip also at WOT...

[donzii22]

ok ive got the same question on a prop for a 1977 18 which i just got, its got a 355cid 300hp with a trs out drive ,currently it came with a 21 pitch cleaver which sucks badly , it only produces 4200 rpm at wot,and only manages a top speed of 48 mph im lookn for more top end speed
and ideas?? txs

[oldLenny]

Donzi22, your numbers seem fine with a 1:50 drive. At 4200 you should see 55 without slip. You are only seeing 13% slip at 48mph with those numbers. I gotta wonder where the other 800 or so rpms went and how much of the 300 HP is still there?

All that being said, you should be in the 65MPH range and 5000 rpm with 300hp with a different prop. Have you tried any others? Is your tach accurate? Is that GPS speed? That 800 rpms you don't have represents about 9mph at 13% slip. What IS the ratio on the TRS.?

[donzii22]

model # on engie is mcm 280, it is 1:50 ratio and all the reading are from the oringal tach and speedo, i have tried a 20 turbo 1 which only gave me about 4300 or so rpms and about 50mph . I'm currently waiting on a 17 picth and a 19 pitch turbo 1 to try out other then that their is not a hole lot of choices is their? txxs

[BUIZILLA]

Are you SURE the tach is accurate?

J

[Rootsy]

and are you sure the secondaries on that 4V are opening??? didn't scot just go through this with his 18???

[donzii22]

cant say for sure the tach is accurate, and as for the secondarys ,when you kick it down it heitsates for a sec then kicks in, it is a little sluggise boat has only had 4 hours put on it it the last 4 years so im sure the carb has some good o varnish it it i fill it with premium , is their a good way to tell if secondarys open up?
and thanks guys for your help

[Barry Phillips]

First of all I think your rpms are high, second I do not think 18s need a 4 blade wheel. The more blades the more drag. I owned an 18 with a bone stock 350 makeing 250 hp turning a 23 Merc Vengeance round ear with a 1.5 gears. This set up ran 65 confirmed at 4800 rpm with no chine walk. The Lazer that your friend is running now is an outboard prop and is lighter than a Mirage Plus for instance. On a light hull like an 18 with 300hp, hole shot should not be an issue. If you can try a Vengeance you might be surprised.

SO-SLO

[donzii22]

so wheres a good place to find a Vengeance propeller?

[donzii22]

ok heres a up date the carb seems to be find secondaries opening ok , did find the ir breather to be restricting things some ,took hatch off and air breather and did some test runs ,boy what a difference got the 5000 rpms very easy . so it looks like the air breather is under sized for it,
but their is no room for taller 1 , maybe i can find 1 same hieght but bigger around.
also the speedo is off by 4mphs. the cleaver sucks big time so tommorrow im getting a 14x19 turbo 1 whichs should help greatly. Plus the more gas shes burning the better shes runnin