More top end speed needed

[jmarty10]

02 Harris 24 ft deck boat. Current prop size is 21p stainless 3 blade. Max rpm is about 4200-4300. Max speed is about 35 mph. If I drop down to a 17p will I get more speed. Seems awfully slow

 

[Scott Danforth]

your currently lugging the motor with a 21p. drop down to a 19p.

is the motor running on all 6 cylinders?

 

[jmarty10]

I should have said it’s a 350 mag 8 cylinder.

 

[Scott Danforth]

then you have a motor problem...the 350 mag should be able to spin a 21p prop on a 24' boat right up to redline

 

[jmarty10]

I was thinking full tuneup to start

 

[Scott Danforth]

check compression
check each cylinder to make sure its fireing
check the exhaust flappers
check the fuel pressure

 

[jimmbo]

24 ft is pretty big for a small block.
What kind of Dead Weight are you hauling around on it? Wife, Kids, Mother in Law, especially the Mother in Law, can/should all be left on shore. Mistresses and Girlfriends can remain aboard. Is the essential weight aboard distributed evenly around the boat? Is the outdrive trimmed properly while underway?

 

[Scott Danforth]

agreed, however my boat is 26' overall and with my flat-tapped small block I could hit 4500 RPM pushing a VP DP290 with B6 props with only 270hp before I played with the motor

a 350 mag with 300hp should hit 4600 RPM without a problem with a 21P prop. same motor in my buddies 240 searay which is about the same size and weight of the Harris.... so unless the OP's boat is waterlogged, pretty sure he is down on power.

 

[jimmbo]

What gearing did the 350Mag use?

 

[Scott Danforth]

1.47:1 if its an alpha

 

[jmarty10]

Yesterday was a full house. 9 people on board. What’s interesting is the boat planes great. Pulled tubers yesterday and it got on plane well.

 

[ahicks]

If it seems to be running smoothly, with no weird issues to speak of, I'd put a 19" prop on it to let the engine turn up a little faster, and take whatever top speed I get.

 

[jmarty10]

Here’s a twist. Went out this morning to check things out. Got up on plane easily but around 3300 rpm and a lot of throttle left the engine would not go any faster but stayed around 3300 rpm. I engaged the throttle completely to wot but no more rpms. I also didn’t hear anymore high Reving when I fully engaged. Did this about 3 times and always the same.
throttle cable?

 

[Maclin]

Seems like more of a fuel delivery issue. Carb not full and leaning out due to internal filter issue or fuel pump issue, or component between the fuel and the pump inlet (anti-siphon valve, pickup in the tank, filter). Could be time for an electrical tuneup as well, cap and rotor and... Sometimes cap/rotor aging allows for crossfire at upper RPM's.

Running off a separate fuel tank would help determine if there is a problem between the fuel and the pump inlet.

 

[jmarty10]

Would a 17 p prop be ok fir my boat? I have plenty of them from my old boat?

 

[jimmbo]

While a 17 would likely fit, it will raise the rpm about 800. with no increase in boat speed, just rocket sled acelleration. As it hasn't been shown what your engines WOT rpm range is, it might also might have the engine over revving.
Another person posting on this thread feels you are not getting full power from your set up and that is should be able to spin your 21 to at least 4600 rpm. Whether or not 4600 is within the recommended rpm range I can't say.

Perhaps a 19"

 

[jmarty10]

Update on this post. Did full tune up. Plugs, cap rotor, and water seperator. Worked great last week. 5200 rpm and 36 mph with same prop. Yesterday couldnt get past 4000 rpm and only 26 mph. Weird thing is at 4000 rpm when i trim up or down the rpms drop and then cant get back to 4000 rpms without fully stoping and getting back up.

 

[SkiGuy1980]

Update on this post. Did full tune up. Plugs, cap rotor, and water seperator. Worked great last week. 5200 rpm and 36 mph with same prop. Yesterday couldnt get past 4000 rpm and only 26 mph. Weird thing is at 4000 rpm when i trim up or down the rpms drop and then cant get back to 4000 rpms without fully stoping and getting back up.

What does your manual recommend for WOT? The hull may be the difference, but I have a 350 MAG (mines MPI, so 300 HP) and WOT range is 4800-5200. Running a 21P aluminum and 4800 RPM it ran 55 mph. That prop would also over-rev and cavitate so badly I'd have to completely stop and start over. I changed to an 18P, 4 blade stainless and greatly improved hole shot and only lost a couple off top end at still around 4800 RPM and lost the tendency to cavitate.

I don't understand how your WOT varies so much (5200 to 4000).

 

[jmarty10]

Interesting you mention hull. Marina guy said the algae growth on the bottom is so bad that after he washed down other boats some picked up 10 mph. But because it always hangs up at 4k rpm its confusing.

 

[jimmbo]

It looks like you have more than one issue happening here.

Potential engine issue
Dirty Hull
Choice of prop

If the hull is dirty it will affect the performance greatly, and unless the boat is planing cleanly, the engine can't get to its top rpm.

regarding engine performance.. What changed between the two days? Load? Weather? Amount of growth on the hull?
Humidity, helping to short out Sparkplug wires?