Hwsii can you run another slip calculation?

[crb478]

I currently have a 1990 Sea Pro 18ft CC with a 1975 Johnson 115 motor. The average weight of the boat loaded is around 2350 lbs and the motor has a 2:1 ratio. The boat has a stepped transom and I am in the process of making a jack plate for it now, hopefully I will be able to get some time in the machine shop over the 4th holiday. I have the motor raised as high as it will go and believe that I still need to go up some, but I would like to get a baseline of where I am at now.

I took it out today in slightly choppy water, but not bad and got a WOT of 5300 rpm and a top gps speed of 30.2mph. I currently have the tilt and trim off, but I did put a level on the bottom keel of the boat and the AV plate and leveled both. I am running a 13.25 X 13 Solas aluminum 4 bladed prop. Can you tell me where I am at now? I realise that I am loosing some from the shape of the lower unit, but I want to get the most out of this set up. Thanks Carl

 

[hwsiii]

Re: Hwsii can you run another slip calculation?

Crb, your prop slip is at 7% right now, and that is about as good as it gets.

Prop Picker

H

 

[crb478]

Re: Hwsii can you run another slip calculation?

Thank you Sir!!!

I think that I will still finish the jack plate since I have already bought the extrusion and will start it about 1/2 inch below my motor height now. I guess to get more speed out of it I will need a motor that will turn a higher pitched prop at a higher rpm. I would have thought that a 115 would have pushed that boat faster. I will try to raise the motor and see if I can get a few more rpm's out of a 3 bladed 17 pitch prop that I have. That should gain me a couple of extra mph. Thanks Again

 

[hwsiii]

Re: Hwsii can you run another slip calculation?

Your numbers are different than mine, I show a base boat weight of 1,900 pounds and by the time you add a 300 # motor, 35 gallons of gas and two people I come up right at 3,000#.
With that weight I find you should be using a 16" pitch prop at about 35 MPH.
How much deadrise is in the boat.

H

 

[crb478]

Re: Hwsii can you run another slip calculation?

I was told from the manufactor that it was 1500lbs empty I added in 300 for the motor 220 for myself 210 for gas and another 100 for batteries and rounded it up to 850 plus the boat. The actual length is 17'6" and the beam is 7'5". It is supposed to have a 15 degree dead rise, again from the manufactor.

 

[hwsiii]

Re: Hwsii can you run another slip calculation?

The manufacturer always knows his boats, sorry I was wrong.

H

 

[crb478]

Re: Hwsii can you run another slip calculation?

I believe that some of the newer models did end up weighing more. They stopped making the boat that I have. The new 18 foot one had a different layout and was longer. Mine is closer to the newer 17 foot model. Last I heard they had been bought out and were no longer being manufactored under the Sea Pro name. Thanks Again for your help!

 

[crb478]

Re: Hwsii can you run another slip calculation?

Can you run it one more time for me? I finished the jack plate and raised the motor about an inch, the boat again is a 1990 Sea-Pro 18 foot CC, 17'5" in actually. Hull weight with out motor reported to be 1500lbs with motor fuel gear and passangers around 2350 to 2500 lbs. I am running a 1975 Johnson 115 with a 2:1 gear ratio and I am running a 13.25 x 13 pitch solas 4 bladed prop. In simular conditons today light chop, but not bad I got to 30.9 mph gps and 5400 not quite 5500 rpm but all most. This was after raising the motor and tweeking the timing. I know I will be close to what I was before but I am curious as to what improvement I may have gotten. I truly believe that I have this 1975 motor maxed out on this boat so I may be a little underpowered for where I would ultimately like to get (35-38 mph running 56-5800rpm). I expect that I just went to a lot of trouble for very little gain, but I guess that is the fun part seeing what you can get out of one. Thanks for the help! Carl

 

[hwsiii]

Re: Hwsii can you run another slip calculation?

Carl, you gained some RPM and some speed but your prop slip rose by .5% to 7.9%. When you get down to trying to get every last little bit of MPH out of your setup and you have the ability to raise the motor as much as you can you might look into a semi cleaver or something along those lines, that 4 blade is designed for pushing heavy loads and is not that great a design for surface running. But if you run the boat in rough seas on a regular basis it definitely holds the boat better and stops you from having to be a throttle jockey all the time. You might try raising it some more though and see if you can get more RPM without it ventilating. I will tell you in the same exact same make and model of prop you can easily find a difference of 1 to 2 MPH at the speeds you are running right now just because of differences in casting and the hand finishing quality that was done on the prop. Many, in fact most props, have a lot of the finishing done by hand at the factory after they are cast, and some are finished better than others in the same finishing processing.



Prop Slip

H

 

[crb478]

Re: Hwsii can you run another slip calculation?

Well I can't go any higher and I might even go down 1/4 inch. I am going to put the tilt and trim back on as soon as I can get another hydraulic line made, Hopefully next week so I may be able to pick up a tad there. The boat is rated for a 140 or 150 depending on who you believe or maybe there were different ratings. I am sticking with the 140 and under. With the stepped transom the boat really backs up much better with the four bladed prop, like night and day better so unless I can pick up a 15 or 13 pitch 3 balde for cheap I will probably keep it. Once I get things set up I generally leave them alone, but I would have thought that this boat would run in the 38-40 mph range before I started, not that I need to go that fast just that it is nice if you ever want to. I would really like to get the motor to the upper rpm range also since I believe that it is easier on them and they last longer if you do, but I am actually fairly satisfied now not perfect but close.Thanks again hopefully I will get done playing with the set up soon get it on a lift and finally rebuild the kicker and actually get to do some fishing.

 

[hwsiii]

Re: Hwsii can you run another slip calculation?

I thought the WOT RPM for this motor was 5,500 RPM and Dhadley recommends 5,800.


H