low RPM

hanzelvh

have now the VP 4.3 engines, WOT is 4000 @ 58 kmph.
Boat weight around 6000 kg.
VP indicates WOT around 4800 - 4900.
Also engines are quite thursty at middle RPM's.
Is this prop situation or can the spark plugs be bad? (or other suggestions?)
I do not know what props I have as it is still in the water.

Michael T

X 

MarkB

Sorry did you say 58 mph at 4000 rpm???  That is huge. What are you worried about?

MarkB

Then that's a little slow. You should get up to 40 - 45 mph.
My guess is it is the prop selection. Bad plugs - you would see that big time in your fuel consumption/mileage.

Mike_G

Did boat always perform like this?  Did you just buy the boat?  Which boat model?  Two engines?  Both same rpm? -=Mike G.

hanzelvh

just have the boat several weeks now.
2 engines, indeed same rpm but the engines are electronically synchronised so if one engine have troubles, the other automatically will reduce to same.

MarkB

How big is the boat? My guess is that they changed the props to get more thrust, which affected top end performance. Like I said, if you had a problematic cylinder, you would see that in mileage for sure. You should be able to get 1.5 to 2.0 mpg.

hanzelvh

boat is 10.5 x 3.5 metres, approx. 6000 kg
indeed I get 2 mpg now, just coming into plane and wot is not as I like it should be.

MarkB

Oh sorry 6000 kg. I thought it was lbs.  That's a big boat. Not sure 4.3L engines can get to 40 mph, but I would expect so. 

How long does it take you to get onto plane from a stand still? 

hanzelvh

it takes about 12 - 15 seconds to get onto plane, speed than around 35 kmph.
This only with full trim tabs, otherwise more time needed.
We were with 2 adults, 50% gas and 75% water.

JC290

Sounds like your props are wrong what type of drives do you have?

hanzelvh

Yanmars??? Never heard about that type of candy bars

hanzelvh

   
props are clean, lowest planing speed is 35 kmph at 3000 rpm, than goes on to 60 kmph at 4000 rpm.
Agree this is not good, will try to have the pitch of my props (sst f4 equals 19") changed to less pitch to get + 4500 rpm.
Launching will be +120 days from now 

aero3113

I had a rpm issue and it turned out that my exhaust flappers broke off and was restricting the exhaust. It's worth a look if your boat has flappers installed.