Added a bobs cav plate and a 'slight cup' to my 15hp 2s merc (10p stainless prop) last week. Before the additions I was getting a steady 18.5kts with two people with 5400 RPM on the tach. Now I'm getting around 4300 RPM on the tach (i did notice it slip to 5000RPM at WOT one or two times) and my speed is only 14-15kts. I also have a jack plate and raised the motor up about 1" from where it was before. Without the additions it was blowing out on turns (outboard at transom height, no jacking up ), and jacked up it was slipping and cavitating. Now it's not slipping or cavitating but the loss of speed and RPM is way more than I thought it would be...? it is still blowing out on turns though for a few seconds. Does it need to come out of the water MORE or less? It's only about 1" above my transom right now...the stock cav plate is about 1-2" above the top of the tunnel in my hull's plane. The outboard has the three tilt settings of 1-3. I have tried them all and settings 2 and 3 seem to be the best when on plane...setting 3 being slightly better due to the hull's running angle.
When you say slight cupping how much are we really talking about? Did you match it up to another prop? You would be suprised how much a little too much cupping would slow a boat down. Do you have more than one prop to test with?
I do not have another to compare to. It is a weedless SS prop (9.25x10) that had 0 cup on it before. I have an 8 pitch non-weedless aluminum prop I can use but going down to an 8 pitch would see a loss in MPH either way so I wouldnt be much of a comparison, right? One thing I have not checked is how high the cav plate is out of the water when WOT to see if it is dragging. I believe it is a couple of inches below the water line at rest right now.
according to a 13:24 gear ratio with a 10 inch pitch you were getting 76.81% prop efficiency before and 75.6% after which based on your speed and rpm loss indicates you added a lot of drag and a little slip. Take the cav plate off and test. If no improvement, take the cup out and put the cav plate back on. If no improvement go back to what you had. 18.5 knots with a 15 pushing two persons sounds pretty good to me. I heard from PowerTech that there are Texans that will intentionally wear down a weedless prop by running in the sand and then add cup, but that was on 70+ HP outboards.
I think your assessment of adding 'drag' is correct. I looked back at the cav plate while running at WOT for a few seconds and it was way under the water shooting out of the tunnel...adding the drag causing the RPM and speed loss this drastic. My jack plate's main 'plate' is current flipped upside down from the previous owner so my vertical adjustment was limited to about 1.5" above transom height. I am flipping it back as we speak and will re-test this weekend with the cav plate further out of the water...
What did you notice being on my skiff? *Bob's Cavitation plate was out of the water with bottom tips just under. *Cavitation Plate is 4" above hull bottom *Manual Jack Plate w/4 3/4" of setback *S.S. Power Tech SRA3 10x12 (slight cup) *Nice little Roster tail *Zero blowout I have yet to see anyone with a Shadow Cast maximize the tunnel and IMO, it's completely useless! Only thing it does well is to funnel grass straight to your prop. I hope Mel offers the 18' without the tunnel. Call up Castaway Customs and see how much it would be to fill in the tunnel. I would bet the performance would greatly increase.
I'm not filling in the tunnel you bum. I do remember how far yours was out of the water, so much that water was coming over the transom.
^That's just spray from the chine. I know of 5 older HB skiffs(Whipray 16 & 17.8) that the new owner had the pocket tunnel filled in just the last two years. Unless you have a tunnel like this: A pocket tunnel is worthless: The ShadowCast tunnel is about the same size as the small HB tunnel mentioned above. In addition to the above advice I would also remove those fixed tabs as well.
My best advice in order to get the maximum performance is to eventually ditch the Bobs manual and go with something like an Atlas mini. Depending on the differences in load at various times, you will find yourself constantly adjusting your plates height. You already have everything positive in your favor: cupped prop, cav plate, tunnel. If you get an adjustable plate on the water, you can tune at the push of a button. Set of electric Lencos would not hurt either. Otherwise, strip it back bare bones, shim and remount the motor without the plate and fish the heck out of it. Sometimes the extras do more harm then good to your performance and also your wallet... Good luck...
