TOPIC: How high??

I'm researching jack plates and chopper props in an attempt to squeak some more speed out of my 16' 1970 Challenger(picture in avatar) with a '70 1350.

I know my question will vary from boat to boat but as a rough estimation, how high can I go before i start to run into cooling issues? I'm currently at about 1.5-2 inches above the keel running a standard bronze 21P prop. That gets me about 47 MPH at 5100-5200ish RPMs(at 3200 feet elevation) but I know this motor and boat is capable of a bit more. Right about 50 at Sea level. I do have a water pressure gauge and at the height I'm at, it has no problems but I can't really go any higher without a jack plate.

A jack plate that sets the motor back will buy me some additional room to raise it and still cool properly, correct? Is there a rule of thumb for how high you can go depending on how far back you go?

Also, what is involved to swap to the lower style water pickup? Lower unit swap?

So many questions...

My thoughts: 6" setback should allow about 1-2" higher with stock water inlets. Not sure, but if available for your gearcase, a Bob's Machine nosecone with low water inlets will help. If not available, maybe a swap to a newer style gearcase. Bob's is a one stop shop with an excellent reputation. Some items are pricey, but top notch. www.bobsmachine.com/

You will get water pressure as long as you keep the bullet of your gearcase under the bottom of your boat and you install water scoop on your standard gearcase. I have run my propshaft level with the bottom of my boat with a standard gearcase with water scoops added. This was around 6 inches off the transom. My boot didnt like it that high, so I lowered it back down to 1 inch below the bottom of the boat. About 5 inches off the trans.


The standard lower unit for the inline was run very high and over 100 mph in J class racing without low water pick ups. I have heard adding a bobs cone actually can slow you down. The extra surface running area makes more drag.

Set back can run your motor deeper in the water if your bow lifts a lot under speed. The motor sits farther back behind the pivot point of your boat, and the pivot point for your boat is the rear running surface. the farther back the jack plate is with bow lift the higher you have to run the motor due to it being pushed down as the bow lifts.

Id get the jackplate and experiment. I bet you could go up 4 inches easy and still get water pressure without scoops. But scoops are super easy to make and install. I plug the top two holes and put scoops on the bottom 6.

Conrad

Good info guys... appreciate it!

Sounds like a jack plate is the place I should start experimenting.

milkdud wrote:

You will get water pressure as long as you keep the bullet of your gearcase under the bottom of your boat and you install water scoop on your standard gearcase. I have run my propshaft level with the bottom of my boat with a standard gearcase with water scoops added. Scoops are super easy to make and install. I plug the top two holes and put scoops on the bottom 6.Conrad

Wow! That is VERY cool Conrad. What is that, 3/16 or 1/4 aluminum plate? How did you shape it, just in a vice or what? Are those stainless bolts that go right through the lower unit? How long are they, and did you have to drill it out to get them in?

Frank

Frank. Its thin aluminum 1 inch by? must be 3/16. yes I just put it in the vise and made a very shallow angle iron out of it. Very shallow is best just have it com out 1/4 inch at most to grab water. This set up asctually tore off as shown @ 74 mph. I have since plugged the top 4 holes and made the scoop shorter and only working on 4 or 5 holes, so it does not get bent withe excess water pressure going by at high speeds.

When i was approaching 3-4 inches off the transom I experienced fluctuating PSI. It was very sensitive to trim angles and would drop at times to 5 psi at WOT. With the scoops Ive seen almost 20 PSI at WOT. it really works

I just used standard 1/4 inch bolts. They fit across fine if you clean the paint off the intake holes. I had to thin out the center portions of my bots to make them work past the exhaust tuner. Very easy to do and install. You could make your water scoops different. Mine is just a quick test and it worked so well I left it on.
Conrad
Conrad

Cool! I could probably make mine out of 3/16 and never have to worry about them. Best I've gotten is 46, but I've got a few tricks up my sleeve I want to try, see if I can get to 47/48. Still no ripping off of scoops though I'm sure!

Thanks Conrad!

Frank

Any setup pictures of your guys' inlines with jack plates would be most appreciated too!

I really enjoy this hull and know it's capable of far more speed than I am currently getting. Even at 50, it's solid, composed and stable and if I could bump that top speed up to say, 60, with minimal setup changes(hopefully just a jackplate, the right prop and address any cooling issues), I'd be thrilled!

Awesome info! Keep it coming!!

I agree. My Power Cat seems to almost run best trimmed out at WOT. That's 45, 46. I bet I could do another 5mph no problem if I could just figure it out!

Frank

I built mine out of 4 inch angle aluminum. But Id buy a ready made one next time. I didnt want 6 inches of set back but I think that would be fine. I would not get farther back than 6. I have to manually raise mine with a car jack on the trailer. Kinda slow

In the following photos the jack plate is down quite a ways. It will go up further.


A few photos of mine.

Conrad