Prop & fuel feed questions/comments

[Anonymous]

> Would anyone have an idea what prop was originally shipped with/is the
> correct one/etc reference an older Shamrock 20' walkaround w/ a 302 Ford
and
> a 1:1 Borg Warner VelvetDrive? It currently has a 13x13 and does not
exceed
> 3400 RPM WOT, this gives a GPS speed ~26 knots.. Yes, we have it trimmed
> correctly. Current common wisdom from mechanical pros is that we should be
> hitting 3800-4200 rpm with healthy engine and proper prop. Compression is
> OK, engine runs good. We recently bought it and it came with a "spare"
12x13
> prop - we're wondering if this what we *should* be using. Rudder and
rudder
> shaft is currently out and off, we're fixing a cracked rudder
port/stuffing
> box and the prop swap would be easier to do now as the rudder and rudder
> shaft have to be removed to change the prop. Comments?
>
> Also, anyone know anything about antisiphon valves re the fuel
> tanks/plumbing and if a problem with it/them being clogged with foreign
> material might lead to occasional temporary fuel starvation symptoms after
> running this vessel long and hard? We've just rerouted the fuel line away
> from risers/exhaust etc. to reduce the possibility of vapor lock being
> contributory. Have not been able to run the boat long enough and hard
enough
> to rule in/out the problem recurring again or if vapor lock was the
problem.
> Last time was several months ago while attempting to get in before a juicy
> squall line could overrun off NE Marathon, FL Squall line 1, us 0. All
> comments are certainly appreciated!
>
>
> -Mike

[Anonymous]

Mike,
I have experienced some of the same problems you have had with my 220 Predator. I had the prop pitch reduced from 14x11 to 14x10 which bumped me up to about 4000 rpm WOT. They (prop shop) can reduce your same prop by 1" which will bump your WOT up 200-300 rpm and put you closer to where you should be. Otherwise, your engine is working way too hard and will not last as long (several mechanics and my new engine manual have emphasized this.

As far as the fuel starvation problem, did this occur when you were getting below 1/4 tank ? I have had my engine cut out in rough seas when the tank I was running off had about 10 gallons or less. As soon as I backed off and kept the boat more level, it kicked back in. I attribute it to sloshing and try to keep from running low on the tank I'm coming home on, if at all possible. I'm not sure if the tank is not baffled enough, or the fuel pickup is not at the low spot, but it has happened more than once.

Good luck,

[Anonymous]

Hmm - that may be a factor. The seethrough filter we have under the dog house is definitely empty when this occurs and the problem has not been specific to one tank or the other. The fuel line had been rather close to the port riser/exhaust and the problem had been somewhat related to being more likely to occur after a 30-45 minute shutdown after running hard. We were thinking a heat soak situation leading to vapor lock.

-Mike

[Anonymous]

A 19' ski boat (2000-2500 lbs) using that same engine would use a 13X13 prop with a 1:1 transmission. Since your boat weighs more (3500-4000 lbs) I would expect a pit less pitch and maybe a larger diameter on your boat.
A ski boat with that prop will go over 40 mph at 4400 rpm (more like 45 mph depending on load).
I was talking to guy across the dock from me about the engine and prop combination on his 1988 20' stalker. His boat also has the 302 ford engine with a 1:1 transmission and a 13 X 13 prop. At WOT boat engine goes 3500 rpm, sounds like it is little over propped like most shamrocks of that vintage

Bob/MD

[Anonymous]

My 1995 20 Pilothouse has the Indmar 302 and Borg Warner 1:1 turning a 13x13 LH 3 blade wheel. It turns 3950rpm @ WOT with a medium load. The factory spec in Shamrock Marine literature was 4000rpm @ WOT. At cruise at 2800rpm, I get about 2.6 mpg. (Nowhere near the 4mpg advertised by Shamrock.) Next spring I'm going to try a lightly cupped 4 blade 12.25x13 wheel to attempt to get another 150 rpm and more thrust at cruise rpm's. I seem to be getting a better result with the 13x13 than others, but it still seems to be a hair too much prop.

[Anonymous]

leprachaun
normal member in standard member. 7. Re:Prop & fuel feed questions/comments....

I had a 1987 20 pilothouse and experienced the same problems.Reduce pitch on the 13" wheel by one inch. Its not good to run a boat over propped. I don't think reducing diameter has the same effect as reducing pitch. You may have increased cavitation. There is a site called marine diesel.com that has a prop calculator that is very accurate. The problem is you must join for $20/year to use it.The fuel starvation problem is probally caused by the anti siphon nipples. I removed mine and found a small corrosion hole. Replace with the same size. Also check the fuel bowl floats.
Date: 07:40 p.m. on 10-05-2000

SCREAMINEAGLE
normal member in standard member. 8. Re:Prop & fuel feed questions/comments....

the anti siphon valves ARE your culprit matey.......romove it put it on the bench...take a small dowel and knock out the ball bearing,spring and collar.Also u may have dirt in your primary AND secondary bowels on the carb..both of these things happened to me lololol and a 13by13 prop is the match for your boat
Date: 08:13 p.m. on 10-05-2000

pmrobert
normal member in standard member. 9. Re:Prop & fuel feed questions/comments....

Thanks, all. I'll soon post the results of Plan "A", replacing the 13x13 prop with a 12x13. A good friend who has been a Borg Vdrive rebuilder in the past was of the opinion that the overpropped situation is really bad for the tranny as well. Hopefully we won't have a cavitation problem with the 12x13. If so, Plan "B" is to repitch the 13x13 to 13x12. Comments and opinions are, as always, well appreciated. I'm trying to not reinvent the wheel, no pun intended....
Date: 06:01 p.m. on 10-07-2000