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1975 Boston Whaler with 25 hp Johnson

J

Jakwi

Member
Hi all,

I recently inherited a 1975 BW with a 2 stroke 25hp Johnson, after a bit of tinkering, and some new parts (condensers, thermostat, coil, impeller, battery cables, and a kill switch) it is running like a champ.

I do have a couple of questions though.

First the restriction plate for the water pickup only has three tiny holes. While I was testing I had that plate removed, and it was pumping quite a lot more water from the muffs. Is there some reason that it is so restricted? It would be pretty easy to drill a dozen more holes and improve the water flow. Thoughts? If nothing else it would definitely rinse better on the muffs. This is pretty important as I'll be using it almost exclusively in brackish water.

Also my top speed on this setup was about 18mph, that seemed a bit slow, a more comfortable rpm produced 15mph. It did jump right up on plane, but I was expecting something in the 25mph range. I checked the prop and it is an 8x10. I do have a 9.2 x 11 that came with the boat. I assume that will make the whole shot a bit tamer, but should improve the top end. What is the consensus? Will that get me to 25mph? or realistically what should I expect from this boat?

Thanks for the help!
 
That plate on the side of the gearcase is NOT the main water pickup. The water pickup is on the leading edge of the exhaust snout, behind the propeller. The channel behind the plate you mentioned connects the pickup to the water pump. Prop wash, plus forward movement through the water force-feeds the pump with an ample supply of water when running in forward gear. Water actually flows OUT of those three small holes under such conditions. So why the three small holes? They provide a water intake when running in reverse, and the prop is sucking water AWAY from the main intake.

Removing the plate, or adding more holes, would allow more water to flow OUT at high speeds, causing an overheat condition. Short story--don't be trying to out-think the engineers that designed the motor. They did know what they were doing.
 
fdrgator said:
That plate on the side of the gearcase is NOT the main water pickup. The water pickup is on the leading edge of the exhaust snout, behind the propeller. The channel behind the plate you mentioned connects the pickup to the water pump. Prop wash, plus forward movement through the water force-feeds the pump with an ample supply of water when running in forward gear. Water actually flows OUT of those three small holes under such conditions. So why the three small holes? They provide a water intake when running in reverse, and the prop is sucking water AWAY from the main intake.

Removing the plate, or adding more holes, would allow more water to flow OUT at high speeds, causing an overheat condition. Short story--don't be trying to out-think the engineers that designed the motor. They did know what they were doing.
Click to expand...


Ah, ok, that makes a lot more sense. I'll have to look at the leading edge of the exhaust snout, I haven't noticed a pickup there, but if that is the standard design then I imagine it is there. Thanks for the help.
 
I suppose that is possible, though it didn't seem very heavy in the water. What can be expected out of these boats normally for speed?

Thanks!
 
That lower unit style was made from the early 1950's to the 1990's -------Those all have the water intake / pick up behind the propeller.
 
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