F36 underpropped?

MDrywa · Post by **MDrywa** » Tue Jan 25, 2011 9:07 pm

My 1976 F36 is only getting a top speed of 22 knots at 4200 RPMs. I normally cruise at 3200 RPM at 13-15 knots. I think I should be getting at least 17 knots at 3200 RPMs.

Here are the specs:
Twin gas engines replaced in 2008 with 300 Mercs
Current props are 18 X 16
The bottom is smooth-soda blasted 3 years ago and barrier coated.

One company suggested upgrading to 18 X 15 four blade props.

Any suggestions would be appriciated

prowlersfish · Post by **prowlersfish** » Tue Jan 25, 2011 10:34 pm

What is your gear ratio ?

your top rpm is fine but the speed seems low .
(upDate) wrong on this top rpm is way low

Is the bottom and running gear clean ?

300 Mercs ? 300hp 454's ?

Conv. or tri cabin ?

MDrywa · Post by **MDrywa** » Wed Jan 26, 2011 6:29 pm

1.91:1 Gear ratio

The bottom and running gear are clean.

350 GMs rated at 300 HP

F36 Conv

I bought this boat last year and the first thing noticed was that the props don't grab like my old F28 Trojan when docking. I have to give it a little gas to get the boat to turn.

prowlersfish · Post by **prowlersfish** » Wed Jan 26, 2011 8:11 pm

I don't think you will like my answer . But here it is . First you are over propped , you should be running 1" less pitch . your engines ( 300HP 350 )should be turning about 5000 rpm at wot. (4800-5200 ) per merc.. The reason it seems not to bite when put into gear is due to the small wheels used due to the high rpm engine , the Engines that came with the boat where rated around 4000 . most likely big blocks rated at 330 hp ( @4000) 2 to 1 gear ( could be 1.91) and running 20x20 props I believe .

what to do ? first make sure the bottom and gear are clean . Cleaned a few months ago don't cut it . Re prop to make full rpm (4800-5200 )
Learn to cruse at a higher rpm if you want to go faster , the are made to run at a higher rpm . want more kick when docking ? go to a 2.57 or taller gear and larger props , not a cheap choice for sure.

over propping is hard on a engine . so what ever you do do not add pitch the over loaded as the are.

Also make sure your tachs are right.

I came up with 18 x15 3blade using a calculator I trust . and also by reading what you have and the rpm you are turning now .

prowlersfish · Post by **prowlersfish** » Wed Jan 26, 2011 8:23 pm

Almost forgot are you using trim tabs ? those 350s are light so the bow may ride high , so using tabs may help

captainmaniac · Post by **captainmaniac** » Wed Jan 26, 2011 8:38 pm

Two other things to consider ... First, any chance the boat is just severely overloaded with crap (or waterlogged hull) and you are carrying too much weight, or bad weight distribution? Second, I have heard that cupped vs non-cupped prop can make a huge difference. My F32 uses a 16x16 cupped (twin Chrysler 383s, putting out 260 horses each).

prowlersfish · Post by **prowlersfish** » Wed Jan 26, 2011 8:55 pm

If he adds cup it will be worst as he can't turn full rpm now

Your engines also should turm 1000 rpm less then his

aweimer · Post by **aweimer** » Sat Jan 29, 2011 6:34 pm

The 1975 manual has them with 2:1 , 350ci Mercs, 20x20 3 blade on the F36 sedan. They are however rated lower in HP though, 255hp. I would go 20x21 possibly with the increased HP. Either i think would be OK. F36 TC is 20x21 on 350ci 255hp mercs.

prowlersfish · Post by **prowlersfish** » Sat Jan 29, 2011 6:56 pm

aweimer wrote:The 1975 manual has them with 2:1 , 350ci Mercs, 20x20 3 blade on the F36 sedan. They are however rated lower in HP though, 255hp. I would go 20x21 possibly with the increased HP. Either i think would be OK. F36 TC is 20x21 on 350ci 255hp mercs.

How could he turn a 20X20 ??? he cant turn a 18 X 16 to full rpm ??
his engines should turn 5000 rpm not 4000

I think you info is mis read as a 440 330 hp (at about 4000 rpm) and a 2 to 1 gear uses a 20X20 or so no way could that be correct

with 20x20 I don't think he wont be able to turn 2500 rpm

aweimer · Post by **aweimer** » Sun Jan 30, 2011 2:41 am

Take a look. Maybe i'm miss reading it.

http://dl.dropbox.com/u/10877359/1975%2 ... 0Sheet.pdf

prowlersfish · Post by **prowlersfish** » Sun Jan 30, 2011 10:23 am

Your reading it right , I ran the numbers and they are on the egdge of over proped but still ok . BUT there is always a but as soon as you load the boat you are over proped all boat makers do this, why you ask ? becuse if they proped it for the real world , then brand x did not brand x can say they cruse faster at x rpm and that can be a big thing for sales.

Back to the boat in question . I stand by what I said . Becuse even thought they are 45 more HP the the 255 , the get that HP at a much higher rpm around 5 grand vs around 4 grand , I looked up 300hp 350 merc engines and found some should be 4800-5200 and the others 4600-5000 , so 4800 rpm is a good number to shoot for , thats a lot of rpm but thats what they are made to turn , I would think they are made to cruse at 3800-4000 (yikes !! ) IF and I repet IF they are proped right and that means to rated rpm .

If this was a high output diesel and was over proped this bad He would have burned a pistion by now

with that said its not good to over prop any engine if you what the most out of it , your over loading it and your not getting all the power its made for and you have paid for .

wowzer52 · Post by **wowzer52** » Sun Jan 30, 2011 10:27 am

I didn't see his engines on that chart. It didn't even show my engines and mine are original stock 233's unless I'm missing it somewhere.

prowlersfish · Post by **prowlersfish** » Sun Jan 30, 2011 10:54 am

wowzer52 wrote:I didn't see his engines on that chart. It didn't even show my engines and mine are original stock 233's unless I'm missing it somewhere.

Its not there ( both ) your engines are on my 76 model price list ( 9/1/75)
your boat maybe a late 75 ,

His boat is a repower done in 2008 , they did not offer a 300 hp 350 back in 1975 , this in new tech stuff

MDrywa · Post by **MDrywa** » Sun Feb 20, 2011 6:00 pm

Thanks for the advise everone. I think we are going start with getting the props scanned to make sure that actually 18 X 16s and go from there

prowlersfish · Post by **prowlersfish** » Sun Feb 20, 2011 7:50 pm

good idea , check the tachs too