Prop and Speed

i have a 14-19 3 blade prop which the manufacturer has confirmed is the original for the boat, this should achieve 30 to 35 knts at 4400-4800rpm i am actually getting 11.5knts at 3300rpm which is max revs
i also have a 14-17 5 blade prop which is increasing speed and revs to 16knts at 4000rpm still nowhere near the proper output,
has anyone any ideas what could be wrong and what to check or change?
would a 14-15 prop be any better?

 

can you tell us about the hull? Weight, beam,length, type of hull ( v -hull,what deadrise) (cathedral hull like 60-70-80'sboston whalers),inboard engine, hp,gas /d.o.? type of transmisssion(prop shaft, z-drive,v- drive) ,Outboard engine,hp, 4 or 2 strokes.This way we'll be able to help you more.

 

The Boat is a Searay 220 DA it weighs dry 4000lb or 1812KG length is 23ft 8in beam 8ft 6in
Planing hull Draft Down 36in up 18in
4.3 mercruiser inboard gas carburretor 205hp with alpha 1 leg
Trim tabs have been used to obtain speeds as described
engine starts, ticks over, revs fine and seems smooth done 450 hours
Holds 58 gallons fuel 20 gals water if it makes a difference
Please let me know if you need anything else

 

continued:

Fantasy, it seems the engine is too small.Seems like others Sea ray 220 DA should have a 5.7 l 260 hp V-8 gas mercruiser.Besides the recent S.R.220 select (loa:22' 11'',beam 8'6'',displ:4000 lbs) has such a power plant.So there is a first problem,I mean it is strange,something happend before you bought the boat....
For now , with 190 brake hp (I checked mercruiser's data for 4.3 l carburettor),this is sure you will hardly reach 18 kts.But I calculated you would reach 18 kts with a 13" x 8" or 8.5" prop. (about the diameter marshmatt and I agree cause he told you,some days ago, to go down by 2" in dia. and you were 15" at that time). About the pitch I am not sure of me right yet.

 

Using common math formulas, and assuming a displacement of 4200lbs (fuel and crew of one) with a 190 bhp (161 @ prop) Max speed is found to be 39 MPH or 34 knots. This assumes all factors are optimum, such as Hull bottom condition, Boat Balance, and Prop.

I would first check motor power, since even if weight was doubled, 161 propHP should provide about 21 knots.

Also speed of 11.5 knots @ 3300 rpm indicates extremely high Prop Slippage (68%). Motor could be Trimmed WAY TOO FAR OUT (assuming this is an IO).

Make sure Drive unit is ALL THE WAY IN, during Start out.

 

Thanks Jango and L.Dosso
i will check compression etc, the engine is original have checked as have orig docs, it also has a 4 barrel carb which increases the hp from 190 to 205, as youve probably guessed im new to boating and this is my first inboard engine, Probably silly question but do they suffer from clutch slip, ive tried the leg up down and every where as well as the trim.

 

I would also check engine timing. If it's too retarded you will be losing a lot of power.

Also check it while running if possible. If the timing mark jumps around alot you could have a bad timing chain although that would likely show up as a bad idle as well

R