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ť Forum Name: PT Boats - General
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ť Topic: Elco 80 ft boats large vs. small screws
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In PT-105 by Dick Keresey the author mentions that at some point while they were in the South Pacific getting ready for war the boats performance suddenly deteriorated and the boats couldn't go faster then about 27 knots.

They tried all sort of things but nothing worked until some civilian engineer suggested using smaller screws. It worked and they never had these problems again.

Any idea why that could be?

Was there something different about the water conditions of the S Pac. as opposed to Elco's home waters?

Were all Elco boats (and other types) thus modified or only those going to the Pacific?

Posted By: Thomas | Posted on: Nov 15, 2018 - 10:16pm
Total Posts: 6 | Joined: Nov 15, 2018 - 7:52pm

I am only guessing here based on my experience, but smaller screws (propellers) allow the engines to run at higher RPM because the load on the engine is decreased with the smaller propellers. Why was it needed? Well, it could have been the heat and humidity, lower quality gasoline, higher operating temperature of the engines due to the hotter ocean water temps.

Former crew member of the Big Blue Sightseer ex-PT 486

Posted By: Gerry McGovern | Posted on: Nov 16, 2018 - 1:11pm
Total Posts: 78 | Joined: Oct 7, 2016 - 1:47pm

As stated above, smaller props place less load on the engines, however I feel the main issue with the lack of power from the engines was a result of the weather conditions in the PTO. The high temperatures and humidity would contribute to a high corrected density altitude. In simple terms, although the boats were obliviously operating at sea level, when you factor in ambient air temp, humidity, air pressure, and even the temperature of the engine room itself it is apparent the engine output was curtailed by a lack of oxygen rich air.
In lay mans terms the boats were operating in an overly " rich" air fuel ratio condition, too much av gas for the amount of oxygen available. A sharp motormac, especially a former "rummie" would be aware of the issue and the corrective measure.

Larry
62 Bel-Air
260 Eagle EXP
79 Cole TR-2
2016 Corvette Z51

Posted By: bubbletop409 | Posted on: Nov 17, 2018 - 4:49pm
Total Posts: 164 | Joined: Apr 22, 2013 - 11:48pm

Thanks guys, that makes sense. What i still don't understand though is how they achieved the same top speeds (by all accounts i read they did, right?) with smaller propellers as opposed to larger ones.

Maybe with the smaller props came a change in pitch and other changes?

Posted By: Thomas | Posted on: Nov 19, 2018 - 8:06am
Total Posts: 6 | Joined: Nov 15, 2018 - 7:52pm

The size of a propeller is relative to the pitch and blade surface. For example:

I had two double-ended (propeller at each end) 100' ferry vessels. They were both the exact same engines and reduction gears producing the same shaft horsepower. They were also both 44" x 30" propellers. However, the 1st boat had a 3 blade 44"x 30" propeller and the other vessel had a 44" x 30" 4 blade high surface propeller.

The 3 blade boat ran at speeds of 9 knots with ease. Coming into the dock, the forward engine would be used for braking/stopping while the aft engine was for steering and pushing forward. It would be able to approach the dock with a full load at 6 knots and stop the boat no problem.

The 4 blade boat would run at a similar speed for the same RPM, but when you approached the dock, the surface area of the 4 blade propeller needed so much more torque to turn that you could stall the engine if you approached the dock at 6 knots and engaged the propeller in forward (just like the other boat).

Pitch and diameter will dictate the amount of torque needed to turn the propeller within the torque curve of the engine. The torque curve of the engine is affected by ambient issues such as heat (warm moist air) versus cold (cool dry air), the quality of the fuel (gasoline), condition of the engine and lastly marine growth on the bottom of the boat.

Taking some diameter off of a propeller can reduce the torque required to get it moving. But once that torque is overcome, the pitch can be sharp enough to be in the right torque for the engine to produce max horsepower within its power-band. Result? If the engine can overcome local ambient conditions and meet the torque required to spin the propellers up, the thrust created should be enough for the vessel to reach hull speed.

Hull speed does not vary by engine or propeller. Hull speed is fixed by the length, beam and shape of the hull. Any engine (gas or diesel) and propeller combination that can produce the thrust needed to push the boat to hull speed will work. You just need to tweak them accordingly.

Former crew member of the Big Blue Sightseer ex-PT 486

Posted By: Gerry McGovern | Posted on: Nov 20, 2018 - 7:40am
Total Posts: 78 | Joined: Oct 7, 2016 - 1:47pm

Thank you for your elaborate explanation, i think i get it now.

Posted By: Thomas | Posted on: Nov 21, 2018 - 9:51am
Total Posts: 6 | Joined: Nov 15, 2018 - 7:52pm