Question for Dahdley or others

[muchco]

I have a 1988 DC175 Larson with an 1987 150XP. I had an earlier post on a manual jack plates that I received some response to as well. I have been doing some reading and have heard that putting jack plate on my pleasure/ski boat will likley make little to no difference, or may even may thins worse. Like blowing out or cavitating on turns. I thought the set back would help from this from not occuring? The jack plate I was going to install has a 4" set back with 4" of lift. I am currently running a 20 pitch raker on the 3 hole (2.25") off the transom at 5200 rpm 50 mph gps.<br />Just looking for a secon opinion. <br />SORRY! in advance if I should not have started a second post.<br />Regards Muchco

 

[Dhadley]

Re: Question for Dahdley or others

Like many other things, a jackplate is a tool. It allows you to do something that can allow your rig to be more efficient.<br /><br />Can it hurt? I guess so, dont know how though. Can it cause cavitation? No, although if you jack the motor up and use a wore out prop with no cup left it may vent some.<br /><br />Generally speaking it provides some added bow lift because of the setback moving the lift point back. It always seems to help get the prop in cleaner water for a better bite.

 

[ob15]

Re: Question for Dahdley or others

I can't answer the jack plate question. The only reason the prop vents in turn is because it is mounted high. Normally when we get our setups tweaked, it is normal to have to trim down a bit when turning to keep the prop from venting. This is the result of some of us wanting to extract as much out of out boat/motor/prop setup as we can. Getting maximum performance out of your boat with and with out skiers might require 2 props. That's what I do.<br /><br />That's the nice thing about jack plates. You can raise the motor up as high as you can for pleasure cruising, fishing, etc and then lower it when pulling a skier. Once you have the height figured out you’re all set. It may take a bit to figure that out but you will eventually.<br /><br />As mentioned before, your rpm’s are too low right now & we need to add about 600 to that. That’s a lot for a jack plate, but Kenny & Dhadley will help you out.<br /><br />Your mention of “2.25” off transom”, does that refer to the AV plate above the bottom of the boat?

 

[muchco]

Re: Question for Dahdley or others

Thanks for the response Dhadley.<br />Dave F. I thank you for your response as well. To answer your question the motor has been moved to the second from last hole on the mounting bracket, which raised the motor 2 3/4" up off the transom.<br />Currently I can trim out the motor to about 80% trim up before it starts to rooster tail. My thought with the jack plate was that 4" of set back would give be more leverage to utilize 100% of the trim.<br />Even if that works that's not going to give me another 600 RPM or at least I doubt it will. I'm still probably looking at going down in pitch.<br />What your thoughts on a 19 Tempest, 19 Trophy Plus or a 18 Raker? Do you have any other suggestions?<br /><br />Muchco

 

[ob15]

Re: Question for Dahdley or others

You setup sounds pretty good (other than the rpm's).<br /><br />I am going to defer setup questions and advice to the experts - Dhadley & Kenny. You may want to revive your old post in the props section or start a new one.<br /><br />Good luck & keep us posted on results.

 

[Dhadley]

Re: Question for Dahdley or others

You're thinking along the right lines. The additional setback will allow some additional lift because the center of gravity or lift point has moved aft. The prop may find some "cleaner" (less aeraiated) water back there too. <br /><br />As for the rooster tail, it's neat but not normally efficient. At least not a huge one. It's not unusual to see fast boats with a small one about 3' high or so. <br /><br />I think your prop choices are good. Lately I've been reading a lot about Trophys cracking / breaking blades. In fact I sold a prop last night to a guy who has a cracked Trophy.<br /><br />Don't overlook the Turbo Lightning's or TXP's.

 

[muchco]

Re: Question for Dahdley or others

Dhadley and Dave thanks for the responses. I will have to wait until spring now to start testing. Today it feels spring is a ways off, it's -35 celsius with the wind up here today.<br />Dhadley do they make the Turbo Lightnigs or TXP's in a shallow enough pitch for me. I looked on there web site and I thought they started at 21 pitch and up?<br />Regards Muchco

 

[muchco]

Re: Question for Dahdley or others

Update: Well I have installed the Jack Plate as well as the Stator that was acting up and a new water pump for good measure. Got out on the lake last night to test. With the new Stator the motor hasn't run so good and the jack plate seemed to make a difference. Water pressure was 6psi @ idle 14 psi @ 3000 rpm and 16 psi WOT. Do these water pressure numbers sound in line? I installed the motor on the plate on the bottom hole and raised the jack plate 1/2 to allow for set back. This left the prop centerline approx 7" below the lowest point on the back of the hull. (Dhadley I will email you some pictures as I am not sure how to attach them on this forum. I used 3 props to start and did not change the the jack plate setting as the water was freezing! The result were as follows: 20 pitch Raker 5500 rpm 52.2 mph GPS 16 PSI water pressure and a decent hole shot. Dhadley remember the 18 pitch 4 blade ski prop? This was my next test. 5700 rpm 50.5 mph GPS 16 PSI water pressure and a wickedly unbelievable hole shot. Lastley I tried a 17 pitch Trophy Plus I had and it just was not enough prop even with the holes plugged and cavitation ring the old 150 would just run away with it. Thoughts and suggestions?

 

[andy6374]

Re: Question for Dahdley or others

The only disadvantage is that if you have a weak transom, which is a reasonable (but not absolute by any means) assumption with a 20 year old boat, and the jackplate/engine combo is going to put alot more stress on the transom and if it is weak you will start seeing many cracks on the transom.<br /><br />Good luck

 

[KCLOST]

Re: Question for Dahdley or others

Your 18 pitch Trophy is right on in regards to WOT rpm...<br /><br />However, I would like to see your water pressure at WOT go up some... Like around 20psi... Try it again at WOT with the motor trimmed down some to ensure that you are not sucking in any air into the intakes...<br /><br />The plate on the lower unit just above the prop, what is its height in relation to the bottom of the hull?

 

[muchco]

Re: Question for Dahdley or others

Thanks for the responses. I will heed the warning regarding the transom and watch it closely. <br />In regards to the plate during the test it would have been approx 1" above the bottom of the hull. The water pressure was 16 psi @ WOT while the motor was trimmed in and trimmed out it did not change.