Deadrise angle and prop position

[nphilbro]

19.6ft fiberglass
20 degree deadrise
Johnson 140
19P

Last weekend I finally got around to hooking up - and using - the 50gal in-hull tank with 10 gallons and removed the temporary 6 gallon tank I had against the transom. The hull tank is positioned roughly 2 1/2ft at back end forward of stern to 7ft front end from the stern and this has drastically changed the hull performance for the better. For the most part it planes a lot drier and the other day I even experienced a very clean ride at 18mph GPS with just 2300 rpm on smooth water (saltwater) with 2 adult men and a child. the AV plate was running just along the top of the water surface.

My question now is that I hear different recommendations on position of the AV plate for changing performance but also hull type is occasionally brought up - as in, this might only be more efficient with a flatter hull. When I set this new (to me) motor on the boat I raised it about 1" (2 mounting holes) so the AV plate is also that distance about the bottom of the transom.

When I give it hard throttle the hole shot is slow, prop seems to slip quite a bit and it seems like the stern is trying to bury itself causing excessive bow rise until it finally flattens out. It doesn't seem as though I can trim this out of it.

As mentioned above, I'm now able to find some sweet spots but I'm beginning to wonder if raising the motor is costing me more performance than I'm gaining.

If I drop the engine to make the AV even with the T-bottom what performance dynamic changes should I pay attention to?

For what it's worth, I'm running a doelfin.

Was I misguided believing raising the engine would enhance performance on a 20 degree deadrise?
Thanks,
np
(I posted some pics a week or so ago in "props" forum before I hooked up the hull tank if you need a pic)

 

[nphilbro]

Re: Deadrise angle and prop position

I need to mention: When shopping for props on various websites 19P is on the extreme high end and that is usually for smaller boats. Top speed with this prop is 36mph at 4700 thru 5000rpm after changing the tank and saw 36mph at 5000rpm with a 15p before changing the tank. Apples and oranges a bit - but thought there might be something there you prop guys might notice.

In this pic the stern is fully weighted but bow is on shore.

 

[steelespike]

Re: Deadrise angle and prop position

Some of your figures aren't right; boat can't go 36 at 5000 with a 15" prop. Is your speed by gps?
If the speed is by gps then very likely you need to verify your tach.
I think the 19 may be too much prop I'll have to do a little research.

 

[nphilbro]

Re: Deadrise angle and prop position

I agree, SS. I'm running into all sorts of goofy here. Absolutely nothing jives and I'm having a heck of a time tuning it in. My speed is via GPS (technically cell towers but I have verified it driving and there are towers all over the vicinity where I was boating).

I suppose my tach could be off. I have another one I could use but I just installed the last one.

What does my raised A/V plate to do performance?


Constant messing with the trim gets the rpms up -to a point - but there is a point of no more speed. Regarding the 15" prop I have two runs on different days with exactly the same result.

 

[steelespike]

Re: Deadrise angle and prop position

What year is the motor,are the props in good shape ?
Your best extreme trim angle will vary based on the prop ,hull and motor setup.
May be less than normal max trim.
The Doelfin could be dragging down top speed and doesn't appear to be doing much for your hole shot.
I would suggest removing the Doelfin until you have the setup refined.Wouldn't hurt to do a run with the 19
without the foil.But it does seem a 19" is too much prop.
Raising the motor can add some rpm and speed.
Usually suggested that the anti vent plate be about even with the bottom of the transom sometimes a little higher.
The less motor in the water the better as long as there are no serious venting issues.
The right prop will allow the most height.You could lower the motor a hole to see how the hole shot is.
From your rpm it appears a 17" prop should get the rpm up a little and help the hole shot.
The boat doesn't appear to be too stern heavy.

 

[nphilbro]

Re: Deadrise angle and prop position

I took it out for run today with just my 9 year old son today. I took the fin off just to see. HUGE difference in performance! It felt like a whole new boat! I even got the top speed up to 41 mph (gps) and this was on a very small lake so I couldn't do much adjustment. The hole shot was way improved, albeit, a still a bit sluggish. The bow settled down a little quicker too.

With my "helper" on board and no one else on the water I was able check the motor at near WOT and the water came right up to the A/V plate.

This is a big boat and the hull has some rough spots I still need to tend to but I'm thinking I'm probably hitting about as close to top speed for that machine as I'm going to get. The transition to plane was still hard to distinguish but the ride was really smooth and stable - it didn't feel like we were going that fast (compared other boats I've been on).

I checked the speed via GPS. My tach isn't working again but I estimate it was near 4700-5k based on past experience. That fin seemed to just suck the life out of it. This boat knows how it wants to ride and attempts to fight it are futile.

I'm OK with the performance now for general lake fishing and hauling using a stainless 19". It calculates to 10-11% slip, which is pretty decent.

I'm going to do another run in the next day or two using my 15" without the fin for comparison purposes.

Overall, I'm just surprised how the doel fins just killed the performance on this hull. I'm sure there are other ways to tweak the performance. Because of the weight of the boat a four blade prop might help with "splitting the difference" between good hole shot, top end, and efficiency.