Confirmation of prop/gear thinking
- Thread starter Al
- Start date
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FF
Guru
- Joined
- Oct 12, 2007
- Messages
- 22,552
500RPM down from WOT is very safe ,I would consider 1800 to be safe , if you can stand the noise and deck angle.
******
"as M1 through M5 with M1 being most heavy duty full time WOT use and at the other end M5 usually at a higher WOT and HP setting light pleasure use with very little WOT use expected. "
There are RATINGS the engine would be run at , only the speed boat M5 would be at WOT , for a very short time.
The engine at M1 rating will still be capable of more HP and RPM , and many will use the short term ratings to go faster ,make a bridge or lock, , by observing the short time limits , no harm happens.
The propulsion engines are usually identical , the owner chooses the M rating he prefers . to operate at .
Pumps and generators may be set up differently at the factory .
******
"as M1 through M5 with M1 being most heavy duty full time WOT use and at the other end M5 usually at a higher WOT and HP setting light pleasure use with very little WOT use expected. "
There are RATINGS the engine would be run at , only the speed boat M5 would be at WOT , for a very short time.
The engine at M1 rating will still be capable of more HP and RPM , and many will use the short term ratings to go faster ,make a bridge or lock, , by observing the short time limits , no harm happens.
The propulsion engines are usually identical , the owner chooses the M rating he prefers . to operate at .
Pumps and generators may be set up differently at the factory .
Nomad Willy
Guru
Al,
Just take the prop down to the guys in Thomas Basin and have them take off some dia or pitch. One inch of pitch off of my then 18X14 gained 200 - to 250 rpm. If it were mine I'd be think'in of cutting off some dia. 20X16 sounds about right but you can probably rely on the Thomas Basin guys (can't remember their business name) having enough experience to know what is best to do. They've been at it since the 50s haven't they?
Just take the prop down to the guys in Thomas Basin and have them take off some dia or pitch. One inch of pitch off of my then 18X14 gained 200 - to 250 rpm. If it were mine I'd be think'in of cutting off some dia. 20X16 sounds about right but you can probably rely on the Thomas Basin guys (can't remember their business name) having enough experience to know what is best to do. They've been at it since the 50s haven't they?
TDunn
Guru
- Joined
- Feb 23, 2015
- Messages
- 889
- Location
- USA
- Vessel Name
- Tortuga
- Vessel Make
- Nunes Brothers Raised Deck Cruiser
The boat requires a certain power to go a given speed. Power is related to fuel burn (1 gph is approximately 19-20 hp), so you shouldn't expect much change in your fuel burn. As far as picking the correct prop is concerned, the method is a LOT more complex than the ultra simple Vicprop formula. Ideally you want to be slightly under propped for flat water conditions. That way when things chop up you won't find yourself grossly over propped. I went through this a a few years ago. This link goes through the entire prop choice procedure in perhaps more detail that you want, but it does explain everything.
Repowering Tortuga - Choosing a new Engine
Repowering Tortuga - Choosing a new Engine
