Derating a 5 hp ( ca, 1995 model) to 4 hp

[Vic.S]

Suppose one wanted to down-rate a 5 hp to 4 hp to get around licensing regulations on certain UK rivers what would be the best way of doing it ?

 

[Chris1956]

A new decal is the cheapest way.

Alternatively, if the authorities are stupid, you could cross out the 5HP and write in 4HP with a marker.

 

[racerone]

Would you need a " permit " to indicate modifications to your motor ?------Then just take a piece of shim stock and make a plate behind the carburetor to reduce air flow.----The hole at 2/3 diameter of what it is now will do it.

 

[Vic.S]

A new decal is the cheapest way.

Alternatively, if the authorities are stupid, you could cross out the 5HP and write in 4HP with a marker.

The question was about how to de-rate the engine. Not about cheating and lying. Some people are not happy doing that.

 

[Vic.S]

Would you need a " permit " to indicate modifications to your motor ?------Then just take a piece of shim stock and make a plate behind the carburetor to reduce air flow.----The hole at 2/3 diameter of what it is now will do it.

A restrictor plate was what I was thinking of.

On the size of the hole my thoughts were that since the aim was to reduce the power to 0.8 of the original the hole would have to be √ 0.8 of the diameter . ie about 0.89 of the diameter,

Reducing it to 2/3 would reduce the area to less than 1/2

 

[racerone]

Do what you like.------Perhaps look at characteristics / calculations of airflow through a hole and engine performance.-----There must be some tutorials on google.

 

[iggyw1]

A restrictor plate was what I was thinking of.

On the size of the hole my thoughts were that since the aim was to reduce the power to 0.8 of the original the hole would have to be √ 0.8 of the diameter . ie about 0.89 of the diameter,

Reducing it to 2/3 would reduce the area to less than 1/2

I may be wrong here, but I think you want to reduce the H.P. by 20% (from 5 H.P. to 4 H.P.). Reduce the hole by 20% and you should have it.

 

[Vic.S]

I may be wrong here, but I think you want to reduce the H.P. by 20% (from 5 H.P. to 4 H.P.). Reduce the hole by 20% and you should have it.

Yes that the same as reducing it to 0.8 of the original unless my maths has become bad with old age

 

[racerone]

But there is wonderful information available to you.----At a certain pressure-Flow through a 1" diameter hole is 4X that of the flow through a 1/2 " diameter hole.-----Nature does not always work the way you think it does !!

 

[Michawilden]

So then you would get your motor derated to 4 hp ( maybe) and then what? How are you going to prove to the authorities that your motor is 4 hp?

You would still be sailing with a 5 hp cowling and 5 hp serial number.

In the Netherlands you have to have a certificate to show that your motor is derated. And they normally to it by restricting the opening of the butterfly valve on the carb.

 

[Vic.S]

But there is wonderful information available to you.----At a certain pressure-Flow through a 1" diameter hole is 4X that of the flow through a 1/2 " diameter hole.-----Nature does not always work the way you think it does !!

Isn't that that what you'd expect on the basis that a 1" diameter hole is 4X as big as a 1/2" diameter hole

 

[jakedaawg]

A restrictor plate was what I was thinking of.

On the size of the hole my thoughts were that since the aim was to reduce the power to 0.8 of the original the hole would have to be √ 0.8 of the diameter . ie about 0.89 of the diameter,

Reducing it to 2/3 would reduce the area to less than 1/2

This assumes that the mass air flow to Kw output is linear. While I am not smart enough to put the reason this is incorrect in writing it is incorrect.

 

[jakedaawg]

Speaking generally. Air intake is a function of engine rpm sometimes combined with valve timing.

For a restricted plate to be properly sized you need to look at a torque curve relative to rpm. You then figure the mass air flow at that point with the out put you are looking for. That would be the theoretical place. Then you would begin testing. Race teams spend tons of money figuring this stuff out.

Not being an engineer i can't explain it further nor is the above complete. It does have the very basic start of what you are trying to accomplish.

The other issue is that horsepower is rated funny. Most manufacturers are not completely truthful and marketing has some influence on the stickers.

 

[kbait]

A different view.. Say your boat currently travels at 5 mph max. A 4 hp would likely push it 4 mph (5 x .8). Block throttle advance so your 5 hp can only achieve 4 mph max (get a free gps phone app for testing). You could use a lead split-shot sinker pinched on throttle cable to block the throttle..or some other creative way. The upsides of this method.. a) easier. b) you could easily show/explain to the Mounties what you have done to effectively max your motor at 4hp output. Good Luck!

 

[racerone]

Folks are confused about boats and motors etc , etc.----I remember the insurance agent asking me how fast my 30' sailboat could go with the motor. .----I said with the right conditions the sails could push it at double the speed of the motor.----Nothing but an amazed look from the agent.----Yes you would have to convince / educate an official on your motor.-----Then a week later another party with a " go by the book " approach might still issue a fine !!

 

[Sea Rider]

If both 4 and 5 HP motors shares similar CC powerheads, mechanically restrict the throttle for the carb/butterfly to open say 3/4 when at full grip, change cowl's number and that's about it..

Happy Boating​

 

[Vic.S]

If both 4 and 5 HP motors shares similar CC powerheads, mechanically restrict the throttle for the carb/butterfly to open say 3/4 when at full grip, change cowl's number and that's about it..

Happy Boating​

No they dont . the 4 hp is 87 cm³. The 5 hp uses the same 164 cm³ power head as the 6 and 8 hp engines