Prop sizes

Justinf89 · Post by **Justinf89** » Thu Jan 01, 2009 8:50 am

Hey guys. I havn't been on here a while because I've been working on my boat, painting, etc. I was looking throught the manuals that it came with and I saw in it that someone who owned the boat before me wrote in it the serial number (BERH0501M77A 335), followed by 330hp, 1:91-1 - 21x20. The boat is a '77 Convertable with PCM 7.4L 330hp motors and 1:91-1 transmissions. The issue is that currently, it only has 17x20 props on it and I was wondering if I should have 21x20 props. Maybe somebody knows what I should have? Thank you, Justin.

Post by **dougl33** » Thu Jan 01, 2009 8:54 am

21x20 is what all the gas powered boats came with prior to 1987.

lobsta1 · Post by **lobsta1** » Thu Jan 01, 2009 8:59 am

Justin, Our boats with the original Merc motors had a 3800 WOT per Merc specs. Bertram test sheet showed that Bert ran them to 3000 WOT. Though rebuilt, I do still have the original Mercs. With the inevitable weight gain ( both us & the boats) & the addition of a hardtop, I found that I had to repitch the props to 21 x 18 to get a 3950 WOT. Al

Justinf89 · Post by **Justinf89** » Thu Jan 01, 2009 9:30 am

I appreciate the quick responses. My motors are rated at 4000-4400 rpm, I think, and I'm getting a top speed of 25.5-26kts. At WOT, it runs 4150 rpm. With my 17x20 props now, do you think that I would benefit by the 21x20 props? Of course, I would have to haul the boat.

IRGuy · Post by **IRGuy** » Thu Jan 01, 2009 9:31 am

Justin... There is a big performance difference between 17" diameter and 21" diameter props. You might well be underpropped and not getting all the performance that is available to you. It would be helpful to the guys here with gas engines to know what your engine RPM vs boatspeed numbers are.. at least the max speed your boat will achieve at WOT, so thay can give you some comparisons to their setups. There is nothing wrong with being a little underpropped, it lets your engines work less hard, but you might be giving away some desireable boatspeed. Frank B - IRGuy@aol.com The weak will inherit only whatever the strong are willing to let them have! "Phoenix" 1983 FBC Cummins 6Bs - 315HP Wilmington, NC

CB35 · Post by **CB35** » Thu Jan 01, 2009 11:21 am

if you are only turning 4150 with 17X20 you are still over propped. You could never turn a much bigger wheel. 21x20 is out of the question. In fact you could loose another inch of pitch and you still may not to get the rpms up to 4400. You might also want to look for the ratio tag on the tranny. It may not be a 1.92:1 or you have got more Shit in that then you really need.

Justinf89 · Post by **Justinf89** » Thu Jan 15, 2009 1:07 pm

Alright, another question. It runs 26kts wot with my 17x20 props. Does that speed sound like it's in the ballpark? I haven't seen many boats this size go faster than that with 454's. Would it be beneficial to get the motors up to 4400rpm's if they are 4000-4400? They wouldn't be working as hard I presume but I don't know how that comes into play with the mileage turning faster either.

Post by **dougl33** » Thu Jan 15, 2009 1:34 pm

The speed sound right, but are you sure the props are 17d and 20p and not the other way around? With only a 17 inch prop and 1.9:1 gear ratio you should be turning way past 4000rpms. I mean like 500-600 rpms past 4K. No offense, but what you're telling us doesn't make any sense. Are you sure: 1. The props are 17 inches in diameter and not 17 inches of pitch? 2. The trans ratios are 1.9:1? 3. That you're getting 4150rpms on the tachs? Have you ever photo-tached the engines? BTW, Since the stated rpm range is 4000-4400rpms, you're fine at 4150rpms.

Justinf89 · Post by **Justinf89** » Thu Jan 15, 2009 4:53 pm

I'm probably wrong. I looked up an old photo of the props and written on there is 20 LH 17 for one of them. I guess that means 17p and 20d? That sounds more correct anyway. I haven't looked at the transmission tag which I'll do this weekend. I know that the tachs are correct because I just installed new faria gauges a couple of months ago, or at least I assume they are. Thanks for the help and sorry for being ignorant...

lobsta1 · Post by **lobsta1** » Thu Jan 15, 2009 6:50 pm

Justin, Don't be so sure about the tachs being correct. My 1978 Bert manual specifically states that the wiring harness is modified to work with the Detmar tachs. I found this out when I replaced all my bridge instruments. My new tachs varied from the Detmars by up to 600 rpm at WOT. Solved the problem by running new wires direct from the bridge to the coils. New tachs are now accurate. Of course none of this may apply to your boat as my distributers were changed by the prior owner to Mallory. Al

Post by **dougl33** » Fri Jan 16, 2009 5:05 am

20d x 17p props seems much more likely with the results you're reporting. Given that you're above 4K rpms at WOT, I wouldn't change a thing.