Porpoise help on 16' Starcraft

[ufm82]

Re: Porpoise help on 16' Starcraft

I agree with walleye- the prop is fine- nice tight fit between the gearcase and the prop. So, that isn't an issue. <br /><br /> I see two transducers and they are both pretty close in. That could be causing an issue like walleye was saying. I would just take them off the bracket and run the boat and see if it does better. Leave the bracket on the boat- just pop the 'ducers off. <br /><br /> I would still raise the engine. It's too low. <br /><br />UFM82

 

[mainline4you]

Re: Porpoise help on 16' Starcraft

Ill rip the transdusers off, raise the motor to 1/2"-1" above the bottom of the boat, and have my wife sit in different places and on the deck while I try this in the water. Ill let you know how it goes.<br /><br />Also about a tach, since this is an 1988 90hp 3 cyl, can I get pretty much any quicksilver tach? There are some used ones around and some on ebay but want to make sure if they are compatable. I was also told to hook it up to the purple wire from the key/throttle box. Is this right? I guess then when I can see the rpms, then I can work on the prop and speed.

 

[walleyehed]

Re: Porpoise help on 16' Starcraft

One change at a time, so you know what works and what don't....<br />as for the tach, purple is power, grey is to sender and a black to ground to complete the circut.

 

[mainline4you]

Re: Porpoise help on 16' Starcraft

Well, I took the boat out tonight for a test run. I raised the motor 2 holes, or 1-1/2" which brought the cav plate about 3/4" above the bottom of the boat. Got her on the water and ran great, but still spun on the hole shot. I stopped, unbolted that big transducer on the right and tried it again. WOW!!! It took right off and came up to plain within 5 seconds and didnt sound like much spin at all if any! The water was bouncy with 1-3 foot rollers so I couldnt give it a complete trial and how the porpoising issue is, but am VERY impressed so far! <br /><br />In these rollers, I was still able to play with the trim a bit to see what it would do. I still could only get to go around 25mph without getting the living crap beat out of me and it was still bouncing, but I hope I can blame a lot of that on the waves.<br /><br />Now its on to the tach. Thanks Walleyehed for the wire colors. Can any "quicksilver" tach work on this motor? I looked at the ones Iboats sells and also some used ones and some on ebay. I just want to make sure that I get the right one without having to pay the $90-$120 at my dealer. (yes I can be a cheep skate)<br /><br />Hey - I cant thank you all enough for the input you have shared! I have learned lots and hopefully more to go with this prop issue and fine-tuning this rig! Hopefully I can find some smooth water soon and let you know how it REALLY rides. Thank you all again very much!

 

[Tacklewasher]

Re: Porpoise help on 16' Starcraft

Can you post a picture showing how high you've got the motor? Just want to compare to my 75 Starcraft Holiday.

 

[rwidman]

Re: Porpoise help on 16' Starcraft

Originally posted by walleyehed:<br /> A full-roller trailer has next to zero support...

Not true. Modern roller trailers have dozens of rollers and provide excellent support. The roller assemblies articulate so that each roller is providing full contact and support to the hull. Unless your boat has a perfectly flat area where the bunks contact it, the support provided by the bunks is less than you might think. Many manufacturers sell their boats with roller trailers and warranty the hull, some for five years, some for a "lifetime". A properly adjusted, quality trailer of either type will provide all the support your boat needs. (Think of the large boats in northern climates that are pulled from the water and left for the winter on land with six or eight stands supporting the hull). The choice is purely one of preference and cost.

 

[cpj]

Re: Porpoise help on 16' Starcraft

bet this is why I lost speed after I mounted my transducer.

 

[bluewater19]

Re: Porpoise help on 16' Starcraft

I have a 17' starcraft almost the same as mainlines and I only have a 75hp on mine I run 39-41 mph varified on the gps. His boat should smoke that easily with a 90 on it. I am running a 18 pitch vengance on mine with the same lower unit as his 90 (2.01/1) He should be running at least a 19 or 20 pitch prop on that boat.

 

[Mark42]

Re: Porpoise help on 16' Starcraft

Originally posted by UFM82:<br /> The anti-cavitation plate needs to be about 1" above the keel of your boat to function properly. <br />UFM82

After reading through this post I ran out and checked my cav plate relationship to the hull. The hull is lapstrake design. The center 18" of the hull are basically flat with a 2" wide keel about 1.5 " high running down the center. The cav plate is level with the flat portion of hull and the keel extends 1.5" below the cav plate. This is as low as the motor goes on this boat.<br /><br />Is this the right setup for my boat? I was going to raise the motor to see if I could get another mile or two pre hour out of her, but now I am unsure.

 

[Moody Blue]

Re: Porpoise help on 16' Starcraft

Glad you got the prop ventilation sorted out. Thought it looked a little too close in the pictures. Now that the hole shot issue is resolved I would try the hyrofoil to minimize or possibly even eliminate the porpoising. Sounds like the motor is high enough now that the fin shouldn't cause any problems.

 

[Dyna]

Re: Porpoise help on 16' Starcraft

My 16' Sea sprite with a 140 I/O will do 37 according to the speedo...but I dont trust that damn thing. It will stop working for no reason & then all of a sudden start working again.<br /><br />Anyways, I had some porposing issues also & played around with moving the ballast onboard around & that didnt do much. Tried different trim angles without much luck also. Then I bought one of the Attwood Hyrdofoils & while I know some here dont like them, I am sold on the things right now. Helped my boat get up on plane quicker as well as at a slower speed & it also just about eliminated all the porposing.

 

[walleyehed]

Re: Porpoise help on 16' Starcraft

If the cav (AV plate) is "in" or "on" the water on plane, the fin "May" help, but when you start raising the engine up, the outer edges of the fin are all that touch the water on a deep-v. This "can" cause some funky handling issues, and in this case, smart-tabs are the best option because it doesn't matter where the engine is mounted, they still do their job continuosly as you run. I have played with a few fins, and they are what they are...but there is a sacrifice.....speed.

 

[mainline4you]

Re: Porpoise help on 16' Starcraft

OK, heres another update on my boat. This weekend I put a hydrofin on the motor and added a tach.<br /><br />The boat still comes to plain very quickly, and the fin did actually help on the porpoising even though its still not completely gone - no matter where I trim it too. It is much better than it was though.<br /><br />NOW - adding this tach gave me some high hopes that maybe it was running at 4500 or so and there would be room for improvement. NOT!!! To my disapointment, it is running at exactly 5300 wot - right where it should be according to the manual. Good thing is that this boat finally made it to a whopping 35 mph according to my gps. I moved weight all over trying a bunch of different combinations and looked at the fin which looked to be skimming on the surfice when crusing.<br /><br />Do you think I should get a different prop like Bluewater has? (a couple posts above) That "vengance" prop is stainless, isn't it? Also I cant really change the pitch since the rpm's are right on, right? I also heard that I should increase the diameter of my prop. It has about a 1-1/4" gap between the blades and the cav plate. I am almost ready to give up on this speed/porpoising issue but would the boat "power through" the porpoising thats left or would it just get worse at higher speeds - and would a same pitch, larger diameter prop help?<br /><br />I will post some pics on what my motor looks like now.<br /><br />Thanks again!

 

[QC]

Re: Porpoise help on 16' Starcraft

If you are looking for more speed and to fix the porpoising for good, lose the hydrofoil and add trim tabs. Probably SmarTabs. The foil may have helped, but now that things are better it actually is slowing you down (sounds like a Catch 22 and it kinda is). Remember, more stuff in the water = lower speed . . .

 

[bluewater19]

Re: Porpoise help on 16' Starcraft

When you say you are running at 5300rpm and you come on plane very fast I say you can turn more pitch. What pitch are your turning now? you say you are using a 19 pitch. My 75 says max rpm is 5250. My prop is stainless but an aluminum prop is ok too. Try a 20 pitch laser II.<br />Good luck

 

[ufm82]

Re: Porpoise help on 16' Starcraft

main,<br /> If you are taching 5300 and that is "exactly what the manual says", you are perfect on pitch, for now. The only way you will get anymore RPM is if you can either #1, trim the engine up to get a better thrust direction which will reduce your wetted surface and reduce drag, #2 reduce the weight of the boat, again achieving less wetted surface, #3 reduce drag in the water, or #4, add horsepower. If you change the prop and add pitch, your speed may go up slightly but your RPMs will drop and lugging an engine can be damaging. <br /><br /> You are running an aluminum prop right now, correct? You may be benefit from a stainless prop- something with some cup that will lift the bow. Maybe a prop that can hold the bow up will do something for you. Keep in mind, changing from an aluminum prop to an SS prop will also boost your RPMs due to the smoother finish and the better hydrodynamics. I think the Laser II idea is great- that prop has LOTS of bow lift and it great when run submerged like you will run it. You can pick one up off E-bay for less than $200 most of the time. Get a Laser II, not an older Laser. The II's have much better blade geometry. <br /><br /> I also agree with ditching the fin and going with tabs. The foils are just plain ugly in my opinion and they do drag quite a bit. I have never liked them although they do work for some people. However, tabs like the Smart Tabs move as the boat's speed increases and that is a major benefit over a fixed plate. I realize that the cost is different but they are worth the investment- they will make a big difference on your boat. <br /><br /> As for the prop again- I will say that I have run the Laser, the Mirage, the Tempest, the Trophy and the Vengeance on my boats- I really like the Tempest and the Laser when I need bow lift- they are excellent speed props that can really make a boat fly. My best speed so far on my current boat (a 19' ProLine center console) has been 53 mph and that was with a 21 Tempest, but I was only turning 5200 RPM. I had 400 more RPM to go. I traded that prop (stupid!) for a 19 Vengeance, hoping to get my RPMs up and maybe the speed just a bit. Well, it was a bad swap as the Vengeance doesn't have the cup or blade area that a Tempest does and it couldn't lift my bow. Porpoising became apparent at 1/2 trim @ 48 mph. I had to trim down to keep the boat under control. I was able to get to 5500 rpm, but the speed was down. I could trim up with the Tempest and actually feel the boat coming up as I trimmed- the ride was noticeably smoother since I had less of the hull in the water. I'm going to go with a 19 Tempest if I can find one that I can swap this Vengeance for. If you can find the Laser II I would buy that before I bought a Vengeance. <br /><br /> Good Luck- keep us posted. I want to hear when that boat breaks the 40 mark. <br /><br />UFM82

 

[stevens]

Re: Porpoise help on 16' Starcraft

Keep in mind, changing from an aluminum prop to an SS prop will also boost your RPMs due to the smoother finish and the better hydrodynamics.

UFM82 - is this always true? Here are the figures I got:<br /><br />23" Black Max, aluminum 3 blade, 36 knots at 4300 rpm<br />21" Ballistic, steel 3 blade, 34,5 knots at 4450 rpm<br />23" Turbo, steel 3 blade, 36,5 knots at 4100 rpm<br /><br />Would this be because the different brands of props have different characteristics (e.g. rake/cup/blade area, etc.) that have a greater influence on RPM than the material they're made from?

 

[walleyehed]

Re: Porpoise help on 16' Starcraft

As stevens has pointed out, most generally figure on a drop in RPM due to higher efficiency.<br />A good SS prop will always drop rpm from use of an aluminum.<br />Notice I say a "Good" SS prop....

 

[ufm82]

Re: Porpoise help on 16' Starcraft

What I meant was that a stainless prop will raise your WOT RPM. That's about the only spec that anyone cares about. Granted, your engine will probably turn at a lower RPM at cruise speeds or somewhere else in the range with the ss prop because of the increased efficiency, but at WOT, you'll get more RPM. For performance issues, that's all that matters. That's also why you can normally bump up a pitch increment when going from an aluminum prop to an ss prop. That's what I was referring to. Also, I'll ignore the "good" prop reference. I don't know if it was meant to be a shot at Mercury props or something else, but I reference Merc props because that is what I have experience with. Main is running a Merc and I thought it would be easier for him to try the factory designed props. <br /><br /> Mark42- you can raise the engine if you want to- I don't know if it will do much for a 40hp engine or not. Keep in mind that the plate is there to prevent air from being sucked in by the prop. I always call it the anti-cav plate but it's actually an anti-ventilation plate. It should be below the surface when the boat is sitting still but you want it to just skim the water when running. If it is underwater while you are runniing, it's just dragging in the water. You have a different issue as you have that keel that can introduce air into your propstream. I don't know if you want to raise the engine or not. What I would do would be to raise the engine one hole at a time until you start to experience problems. Keep in mind, if you are doing the testing by yourself and raise it, it might be too high the next time you have two people riding in the front of the boat. It will vary depending on your boat's attitude. But, if you want to try it, it's free! That's the good part. LOL<br /><br />UFM82

 

[stevens]

Re: Porpoise help on 16' Starcraft

Those figures I posted above were indeed WOT figures, for a Mercury 120 hp diesel.<br /><br />I guess prop characteristics are very different from one brand to another, and the only way to get it exactly right for your specific boat is to try some out.