Power loss

[Chica]

Last year I had a problem with caviatation or ventilation, I'm not clear on the difference, but thought I had this improved with raising my two transducers which were mounted too low in the water. After doing alittle reading I though I might help my remaining caviatation/ ventilation with a composite four blade prop and a peratrim hydrofoil. Well, yesterday I took Chica out to a local reservoir for a shakedown cruise before traveling to Vancouver Island for the yearly salmon trip. I had anticipated some speed loss with the new additions, but was surprised to find out just how much of the top end I had lost. On the Island, with trim tab, powertrim, and in the "sweet spot" I could get 33, 34, and sometimes 35mph. Yesterday, top end was 26mph. Now I realize I was operatating at 3500 feet above sealevel, but felt this was a significant loss. When I would try to bring the bow up, the paratrim really dug in and power loss really went up. My plan is to not change anything until I get to sealevel, and then remove one addition at a time. Your thought, CHICA

[Sea Wolf]

CHICA

It would not be out of the question for the various factors to add up to that much loss, but it does seem a little excessive.



For informational purposes, it might help to know the rpm differences along with the changes in pitch and diameter between the props.

I can't really remember the formula for the percentage of performance loss per thousand feet in elevation gained, but if a gain in 3500 feet were equivalent to 10%, that would account for 3 mph of the difference.

The Comprop's surface is pretty rough, and frictional losses there might account for 2 mph.

Then there's the Permatrim, and although its probably the best hydrofoil available for the C-Dory, another 1 mph loss due to friction might be accounted for there.

You're right! The best thing to do now is to go out at sea level and check out the factors one at a time. Will be interesting to just run the boat as is and see what difference the elevation change makes. Then change props for another trial; any remaining difference would be due to the Permatrim, all else being equal. Let us know how the test comes out! Joe.

[Redƒox]

Loss up to 3-MPH is a reality with hydro's. I'd like to hear some feed-back on them new-fangled-dudes (Permatrim) I would like a hydro' that does NOT produce LIFT Not good for the single-engine handling caracteristics on my Baby

[Larry H]

I think altitude loss is 3 % for every 1,000 ft above sea level.

[Forty Two]

On my old 17' Arima with a Honda 90, there was a 5 mph difference between a 4-blade Comp Prop and a 3 blade stainless prop - 30 vs 35 mph at the same 5500 RPM. I don't know why there is such a difference, but I think it is significant. Of course the Comp Prop was about $75 new at the time, and the stainless prop was about $400 new - but I got the stainless prop used for about half that.

[Casey]

We expect to depart today, headed for the Blakely Gathering. Having installed a PermaTrim last week, and this will be the first test for it. Didn't change prop's (ProPulse graphite) as I didn't want to induce too many changes lest the changes in performance get hidden. I've kept pretty careful performance records for speed and fuel consumption so hope to have some pertinent info before long regarding the PermaTrim.

As for the effect of altitude, you're correct: 3% performance loss for every thousand feet. The first time we went to Lake Powell (3500MSL) I was amazed at how much the altitude affected Naknek's performance. Boats with lots of raw power don't really notice the impact, but in a C-Dory where we're operating without hundreds of extra "horses" the effect is easily seen for sure!

Casey
C-Dory Naknek

[Aiviq]

Do keep us posted on performance changes with the permatrim.

FYI: The original U.S. permatrim distributor, John Jenson, has retired from the permatrim business. The new U.S. distributor is an outfit called Shipyard Island Marina, 1-800-213-3323. www.shipyardisland.com. The point of contacts name is Andy Munao. He seems very prompt and helpful (as was John Jenson).

Jim