Engine lack of speed?

[BruceB]

Had a chat with a new start at work today and he has a bit of a puzzler, boats a bit slow to what I would have thought.........
He has a 5.2m Sparrowhawk with a 3 year old 90hp rood 2 smoke on the back, very little kit apart from couple of cans of fuel. He gets 32knts flat out at 5500rpm using a 17" prop.
I was a bit surprised at this thinking he would see somewhere in the region of 40knts with that hull and motor and he would be swinging a bigger prop, probably a 21" for the boat size and weight.
Anybody any ideas? I haven't seen the boat yet but something doesn't seem right there..........

[Bigmuz7]

looks ok to me .. 21 would kill it imo

[BruceB]

My large heavy 90 Soozi is swinging a 19" prop at 5500rpm and approx 35knts, my hull is a bit longer, heavier and wider than his.
With his lighter weight, less beam and shorter hull using a lighter motor as well as less kit I would expect his to be considerably faster than mine, not slower.
Or maybe once I hook up the GPS I am not going anywhere near as fast as I think? Certainly my old 5.3m humber did 27-28 on a 60hp swinging a 17" and was qiute heavily laden with 82lts of fuel and lots of spares.
I'll need to go and try now?
What do you get on your flat out?

[Chris]

Originally Posted by BruceB

Had a chat with a new start at work today and he has a bit of a puzzler, boats a bit slow to what I would have thought.........
He has a 5.2m Sparrowhawk with a 3 year old 90hp rood 2 smoke on the back, very little kit apart from couple of cans of fuel. He gets 32knts flat out at 5500rpm using a 17" prop.
I was a bit surprised at this thinking he would see somewhere in the region of 40knts with that hull and motor and he would be swinging a bigger prop, probably a 21" for the boat size and weight.
Anybody any ideas? I haven't seen the boat yet but something doesn't seem right there..........

I used to get 37 knots in a 5.25 Viper with a 90HP Johnson v4 on the back.

Sounds a bit slow to me, probably need to look at his engine height on the transom. Anti Ventilation plate needs to be a couple of inches above the bottom of the transom I would have thought.

[Bigmuz7]

Originally Posted by BruceB

My large heavy 90 Soozi is swinging a 19" prop at 5500rpm and approx 35knts, my hull is a bit longer, heavier and wider than his.
With his lighter weight, less beam and shorter hull using a lighter motor as well as less kit I would expect his to be considerably faster than mine, not slower.
Or maybe once I hook up the GPS I am not going anywhere near as fast as I think? Certainly my old 5.3m humber did 27-28 on a 60hp swinging a 17" and was qiute heavily laden with 82lts of fuel and lots of spares.
I'll need to go and try now?
What do you get on your flat out?

34 -35 Knots with a 5.7 hull, with a 13x19 alloy job,.. so ok he might get a couple of knots from somewhere, but is it really worth it ? Chris might be right with the height thing, cos that can sure make a difference, as I have found out over the years. BTW I tried a 21 x 13.25 stainless and it killed my Honda stone dead , think I got 3500 rpm tops, 3 up & 100 litres of go go juice

[BruceB]

I haven't seen the boat yet so I am also wondering on the engine height but a few extra knots are always worth it to try and find if they don't cost you anything.
My aim in hoping for 35 knt WOT is that I should be able to cruise 28-30 at a reasonable fuel consumption. The faster the better to cover the sometime fair distances around here.
If he managed to get his up to 37-39knts they he could probably cruise around 32 or so.
I still think 32 flat out with a 90 on a small 5.2m Sparrowhawk is pretty poor.