engine horsepower

[oldmanofthehills]

deleted

 

[PetiteFleur]

Ignore!!

 

[Baccarat]

? If the OP is still out there I would be grateful to hear from him. I understand the boat in question (called Satsuma) is now back in this country.

 

[pvb]

? If the OP is still out there I would be grateful to hear from him. I understand the boat in question (called Satsuma) is now back in this country.

Welcome to the forums!

Resurrecting a very old thread is fraught with problems. For starters, many of the people who posted in the thread might no longer be active members. For that reason alone, it's always best to start a new thread if you have a specific query.

In this case, the OP was stuartgsd14, who hasn't been online since March 2012.

The poster "satsuma" hasn't posted since 2013 and hasn't been online since April 2020, so you may not get a sensible reply.

 

[Baccarat]

Welcome to the forums!

Resurrecting a very old thread is fraught with problems. For starters, many of the people who posted in the thread might no longer be active members. For that reason alone, it's always best to start a new thread if you have a specific query.

In this case, the OP was stuartgsd14, who hasn't been online since March 2012.

The poster "satsuma" hasn't posted since 2013 and hasn't been online since April 2020, so you may not get a sensible reply.

Thanks, all noted for the future.

 

[RivalRedwing]

I realise a long time after your post, but what diameter and pitch prop was that with?
I also have a 25.5 with a folder and a 10hp engine and at present can’t get close to 7 knots.

Whats your length on the waterline - 7 knots from a 25.5 foot boat sounds likely to be more than the theoretical hull speed (25 ft on the waterline has a theoretical hull speed of 6.7 knots using 1,34 * sq root of LWL), I suspect there is a fisgerman's tale or a tide in here somewhere.

 

[Baccarat]

Whats your length on the waterline - 7 knots from a 25.5 foot boat sounds likely to be more than the theoretical hull speed (25 ft on the waterline has a theoretical hull speed of 6.7 knots using 1,34 * sq root of LWL), I suspect there is a fisgerman's tale or a tide in here somewhere.

Thanks - you are right. These boats have a waterline length of 21’ 3” giving hull speed of 6.16 knots. I currently get 5 knots, hence even if the OP is only in fact getting hull speed I would be interested to know the details of the prop and gearbox ratio given that the engine power on the 2 boats is virtually the same.

 

[LONG_KEELER]

Thanks - you are right. These boats have a waterline length of 21’ 3” giving hull speed of 6.16 knots. I currently get 5 knots, hence even if the OP is only in fact getting hull speed I would be interested to know the details of the prop and gearbox ratio given that the engine power on the 2 boats is virtually the same.

I've never really obtained theoretical hull speed under power much.

Under sail yes. Perhaps the squatting under power, but more even spread of power when under sail has something to do with it.

 

[oldmanofthehills]

I've never really obtained theoretical hull speed under power much.

Under sail yes. Perhaps the squatting under power, but more even spread of power when under sail has something to do with it.

The hull speed is when the disturbance from the bow wave drags exactly at the stern seriously preventing easy increase in speed. Normal formula for speed in knots is Square root (2 times waterline length) though variants exists using a slightly different constant. Obviously a slippery stern helps get up to the limit hence the old principle of Cod Head/Mackerel Stern, and Cats have less in the water so can power through that limit. However enough power and the limit can be transended - a bit like the speed of sounds for planes.

Squatting under power wont help, but the prop is normally set for optimum effect at a certain speed then becoming inefficient at greater speed - greater than notional theoretical might be easier under sail

 

[Spyro]

Thanks - you are right. These boats have a waterline length of 21’ 3” giving hull speed of 6.16 knots. I currently get 5 knots, hence even if the OP is only in fact getting hull speed I would be interested to know the details of the prop and gearbox ratio given that the engine power on the 2 boats is virtually the same.

Your calculations are a bit off on that one. It's nearer 5.3 knots. max hull speed.

 

[Baccarat]

I've never really obtained theoretical hull speed under power much.

Under sail yes. Perhaps the squatting under power, but more even spread of power when under sail has something to do with it.

Thanks - interesting

 

[Baccarat]

The hull speed is when the disturbance from the bow wave drags exactly at the stern seriously preventing easy increase in speed. Normal formula for speed in knots is Square root (2 times waterline length) though variants exists using a slightly different constant. Obviously a slippery stern helps get up to the limit hence the old principle of Cod Head/Mackerel Stern, and Cats have less in the water so can power through that limit. However enough power and the limit can be transended - a bit like the speed of sounds for planes.

Squatting under power wont help, but the prop is normally set for optimum effect at a certain speed then becoming inefficient at greater speed - greater than notional theoretical might be easier under sail

Thanks - the formula you quote gives 6.5 knots in this case. These boats have a slippery stern which will help.

 

[Baccarat]

Your calculations are a bit off on that one. It's nearer 5.3 knots. max hull speed.

Thanks, are you using a different formula to get to 5.3 knots? - my calculator advises that the 2 formulae previously given both result in more than 6 knots.

 

[TernVI]

Thanks - the formula you quote gives 6.5 knots in this case. These boats have a slippery stern which will help.

Actually, the broader stern generally increases the hull speed.
Back in the days of 'cod head mackerel tail' design for sailing ships, they used a guide of sqrt of 1x lwl in feet.
'Modern' yacht design has pushed the multiple to ~1.4 i.e. sqrt (2x lwl)
Modern probably means something like post 1970s in this context though.

A lot of classic yacht designs are all about increasing sailing waterline as the boat heels.

But hull speed is not a 'brick wall', it's an arbitrary point on a drag/speed graph which tends to be curving upwards in that area.

 

[Baccarat]

Actually, the broader stern generally increases the hull speed.
Back in the days of 'cod head mackerel tail' design for sailing ships, they used a guide of sqrt of 1x lwl in feet.
'Modern' yacht design has pushed the multiple to ~1.4 i.e. sqrt (2x lwl)
Modern probably means something like post 1970s in this context though.

A lot of classic yacht designs are all about increasing sailing waterline as the boat heels.

But hull speed is not a 'brick wall', it's an arbitrary point on a drag/speed graph which tends to be curving upwards in that area.

Thanks - the Hustler 25.5 has quite a broad stern for an early 1970s design, which narrows to a shallow skeg at the waterline so I would think a relatively efficient low drag stern which would optimise speed under power.

 

[Spyro]

Thanks, are you using a different formula to get to 5.3 knots? - my calculator advises that the 2 formulae previously given both result in more than 6 knots.

Sorry my mistake.

 

[Baccarat]

Sorry my mistake.

No worries!