Prop size??

[Ahzzy]

I can't seem to find what the recommended prop size is for my boat. I know it all depends and is varied on how you are using your boat.

I have a 1999 Larson SEi 186 with a Volvo Penta 4.3GS 17'11". Right now I'm running a 14.5 x 19 pitch 3 blade aluminum. I get out of the hole quick because my girlfriend and I wakeboard which is what I want. But I'm topping out at 30-35 mph and 3800rpm WOT. Does this sound like a prop issue if I'm trying to cruise a little faster and I feel like the RPMs are low.

Would it matter if I switched to a 4 blade prop?
Is there a recommended size for my boat?

 

[Maclin]

Sounds more like the engine is not at full power potential, as it is not hitting near the top RPM's it should be at wide open throttle under load. A 4.3 in that size boat should rev over 4200 minimum on the way to 4600 max, and be hitting 40-45 mph. Are you sure the engine is in good condition, in good state of tune? The 19 pitch prop seems to be about right if your engine could rev to the 4500 rpm's that it should be, and your speed would be over 40.

When was the last tuneup? You may need to do a compression check also, to see what general condition the engine is in.

 

[Maclin]

Oops, manners...

Welcome Aboard!

 

[Leardriver]

Near sea level, as opposed to 5000 feet plus?

 

[Sea Rider]

Is that a first, second, third hand combo ?

Happy Boating

 

[Ahzzy]

Sounds more like the engine is not at full power potential, as it is not hitting near the top RPM's it should be at wide open throttle under load. A 4.3 in that size boat should rev over 4200 minimum on the way to 4600 max, and be hitting 40-45 mph. Are you sure the engine is in good condition, in good state of tune? The 19 pitch prop seems to be about right if your engine could rev to the 4500 rpm's that it should be, and your speed would be over 40.

When was the last tuneup? You may need to do a compression check also, to see what general condition the engine is in.

To be honest I bought the boat from a guy I know who is a licensed dealer and boat mechanic. He said the engine was rebuilt the year before he got it and he's had it for 2 years. I looked up Volvos website and it said top RPMs should be around 4400-4800.

This is my first boat ever I haven't done a compression test yet When I full throttle to get out of the hole some times the engine sounds like it might struggle a bit. Almost like it has a heavy load but there is probably only 900 lbs on board. I am not sure what step to take. I had the spark plugs changed and the oil is clean. Could the carb need cleaned? It idles real smooth.

 

[Ahzzy]

Is that a first, second, third hand combo ?

Happy Boating

I'm sorry I don't know what you mean here.

 

[Ahzzy]

Near sea level, as opposed to 5000 feet plus?[/

I'm at 800 ft above sea level on a lake.

 

[Maclin]

I think the engine needs to be checked. Things like valve lash can limit rpm's when not set properly. Since this is on a still new rebuild/refresh it should be checked out. A compression check is first order. They are fairly simple in nature, most depends on how easy or not it is to get to the spark plugs. Most shops will offer this at hourly rate, some have a flat fee for it, that type of thing.

Basics before you start::

• Tools needed, main one is a compression pressure gauge with a screw-in adapter that goes into the spark plug hole.
• Battery at tip-top of charged.
• Muffs or other method of supplying water to engine cooling system.

Here is a basic outline of what happens:

• Spark plugs are removed (all of them).
• Throttle is moved to wide open, needs to stay that way all during the engine cranking and gauge readings.
• Water needs to be supplied to the engine any and every time the engine is cranked.
• End of wire from the coil grounded to keep volts from jumping out the ends of the individual plug wires or end of the coil.
• Screw in adapter to first spark plug hole and crank the engine over several times until gauge stops moving up, that is the reading for that cylinder.
• Move to next cylinder, repeat, record each reading by cylinder number, usually cast into the intake manifold right on top of each port.

After the compression is known then the troubleshooting can begin in earnest.

 

[Maclin]

Compression gauges are available for loan or rent from the larger Auto supply chains.

 

[Maclin]

Are you familiar with using the trim functions?

Normal takeoff is with trim all the way down, then as the boat gets up over that initial wave and onto plane the trim is bumped up slowly until a porpoising is felt, then backed down a little. If you are not trimming up on plane then it may be plowing some and place more load on the engine, and that might account for some of the low rpm's you are seeing. If your trim gauge works pay attention to it for next time so you don't have to go all the way up into a porpoise situation

Do you know if that boat come new with the 4.3 size engine? I am just wondering about the drive ratio. There is a drive ratio plate on the outside of the drive on port side just about in the middle. If it was original a 4.3 then it will be 1.78 I think. See if you can find that, and is legible.

 

[Ahzzy]

Are you familiar with using the trim functions?

Normal takeoff is with trim all the way down, then as the boat gets up over that initial wave and onto plane the trim is bumped up slowly until a porpoising is felt, then backed down a little. If you are not trimming up on plane then it may be plowing some and place more load on the engine, and that might account for some of the low rpm's you are seeing. If your trim gauge works pay attention to it for next time so you don't have to go all the way up into a porpoise situation

Do you know if that boat come new with the 4.3 size engine? I am just wondering about the drive ratio. There is a drive ratio plate on the outside of the drive on port side just about in the middle. If it was original a 4.3 then it will be 1.78 I think. See if you can find that, and is legible.

I'll try the trim functions. I never did that before. I was going to try a compression test either today or tomorrow. It has about 150 hours I would say since the rebuild. The ratio I can read from the serial tag is 166 it says. Not 100% sure if it was originally a 4.3 but I believe it was. Outdrive is a SX. Whenever I plane I should trim up a little correct weather or not in full throttle?

Thank you for all the advice!!

 

[Maclin]

Yes, on plane need to trim up even just cruising. You should be able to either go faster at same throttle, or be able to back off throttle some if you want to stay at same speed.

 

[Maclin]

One of the reasons to trim up even at the slower planing speeds is you can not pick up bow steer from wave action, can get uncomfortable.

 

[muskyfins]

Great info Maclin.:clap2:

another thing a lot of folks around say is to use a calibrated test tach to verify engine rpm.

 

[Ahzzy]

Yes, on plane need to trim up even just cruising. You should be able to either go faster at same throttle, or be able to back off throttle some if you want to stay at same speed.

Ok thank you for the info that's awesome! I'll try that next for sure. But won't have the boat out again till Tuesday

 

[Ahzzy]

Great info Maclin.:clap2:

another thing a lot of folks around say is to use a calibrated test tach to verify engine rpm.

I'll see if I can find one to use.

 

[Sea Rider]

Was asking if it was a pre owned combo as previous owner could have stayed with delivered prop or may have changed it for a less pitch prop which isn?t known.

Need to make engine to run at least liddle to max of it's 4400-4800 wot rpm range, thant's at 800 ft of elevation with a tach doing it's homework right.

At 3800 OB is 1 K off from it's max wot range, at altitude will need in theory going 5 lesss pitches if +200 wot revs applies going one less in pitch ? if at sea level 3-4 less pitch with 3 blade props.

Don't know why boaters recommends having trim down when getting out of the hole, that position will make the bow to plow down on water while aft side of boat rises losing initial good speed to plane out. With deck weight evenly distributed try slight trim up, go full throttle, once on plane trim down to neutral trim if that trim position makes combo to ride parallel to water surface...

Happy Boating

 

[Maclin]

" that position will make the bow to plow down on water while aft side of boat rises"

Nope.

 

[Sea Rider]

Are you 101% positively sure ?....this is what boats/engines manufacturers recommends, have check them all...


Also applies when out of the hole...

Happy Boating