rpms, props, mph

[a1machinista1]

Hello all and thanks for any help in advance. I need some info so I can research different set ups in an attempt to find the "sweet spot" in our boat. It is a 1992 seaswirl spyder 20' 6". It has a direct drive 351 commander with a 1:1 velvet drive.. We took the boat out today and it ran great. not counting a little linkage adjustment to start the day off. Just a note to whoever may have info for me... Everything in this boat is new in the last 2 months. stringers, floors, engine box, upholstery, carpet. engine has been thoroughly checked out and gone through, Including new carb, filters, water seperator, fuel line, plugs, wires, manifolds, all hoses. Etc.... I say this incase someone has the erge to tell me to check the spark plugs ...With 2 speedos that could not agree, I figure the boat topped out at 37ish at 3100rpm!!! The engine is running very strong,I do not know how to figure the pitch on the prop? To be honest I didnt look when we got home!!! Is is stamped? if so where usually?? It is a 3 blade with what I would guess is a very steep pitch??? Can someone give me some basic math formulas and maybe a little education or prop pitch, prop size, blade count and how these things affect performance of the boat. What is a good rpm operating range in this set up?

 

[a1machinista1]

Re: rpms, props, mph

Ok I just looked and heres what she says..

CUP 9 87
13LH13 FEDERAL
HI-BRAI

 

[steelespike]

Re: rpms, props, mph

Your prop is 13" in diameter left hand rotation 13" pitch.It appears it has cup info which I can't help you with except to say it improves the grip on the water and may influence hole shot, speed and or ability to stay on plane. Looking at the prop from behind it rotates counter clockwise.Sea swirl Spyder sounds like a fast boat,by the seat of my pants a 13 X 13 should run
close to the max rpm for the motor I'm guessing 4,800-5,000.Disregarding slip 13 means the prop moves forward 13" with each revolution. multiply by 3100 for inches in a minute. X 60, devided by 12 for feet devided by 5,280 = mph
Your 1700 rpm short and imo about 13+ mph short you can't make that up with a prop. Unless someone has ruined your prop with some poorly executed modifications.
So if indeed the motor a A+ the only thing left is the hull.Does it get up on the water
bow up not laying down flat like its glued to the water?If it has tabs have you tried raising them clear of the water?

 

[a1machinista1]

Re: rpms, props, mph

OK, I guess the first thing that gets me thinking is the glued comment. Last year we had a 17 'bayliner capri and it was a nice boat, ran great.. When it took off the bow rose so much you could'nt see over the boat.. This boat lays pretty damn flat when it is getting to plane. I just figured it is boat design. is this correct??? lay flat? Last night I noticed one of the blades has a couple knicks in it. Besides that it looks like it is in good shape... I guess I should take it out after work and verify rpms... and mph

 

[steelespike]

Re: rpms, props, mph

I can't speak specifically to the Sea Swirl but the typical straight drive boat the bow would come up some probably not like an I/O but yes it would rise and at speed
the boat would ride on the rear 1/2 to a 1/3 of the bottom. Spyder sounds like sporty so probably a fast bottom perhaps even running out flat at the stern.
It is about 3,800 lbs but I believe the base motor was 240hp. I had a 60 year old
Mohagany boat that was 4,000lbw a reasonably fast bottom and 125hp at 3,200 rpm it would run 31 mph on old fashioned(1929) prop technology. about 33-34 with a modern prop circa 1935.