confused on prop change

[MBAKER]

Help me understand this?.2001 Maxum 2100 with merc 5.0. rpm range should be 4400-4800 WOT.

With an older Merc 48-78120 19P prop 4 people and our normal load of equipment, skis etc it would only run 4000-4100. Trimed full down.
Changed to a new powertech 17P same load 42 maybe4300 rpm.

I was expecting a bigger jump with the new prop and get up in the range but it didn?t. Im assuming that maybe the merc was a outdated prop, and the powertech maybe has a better blade design, more cup etc I don?t know. I just thought it would have increased more than that. Going by this data seems like a 15P is prob in line. Only thing I question is whether that seems small. But it?s a pretty big heavy boat even for the 5.0, and I believe it has a 1.47 drive gear in it, where most literature says the 5.0 usually got a 1.62 in those years. So maybe that?s why the prop seems small to me compared to other similar boats.

Also the previous owner put a ?hydrofoil? on it, and I know there are mixed results with these. If I understand right thesea re designed to help lift the boat on takeoff, but in theory should be nearly out of the wateron place, is that correct? If so I noticed that this one is 6? or so underwater still. Do you think its possible that its creating enough drag that it hurts performance?

Seems to be in good tune, although im going to recheck the timing again this weekend as the light I used last time seemed erratic, and prob put new plugs in it check it over etc.

Any thoughts?

 

[steelespike]

Re: confused on prop change

No question the foil is killing your performance.Lose the foil and see what your numbers are with both props,and get back to us.We really need your speed as well.
No doubt your hole shot will be worse but we can work on that.
When replacing plugs note their location and condition.Be sure the throttle plates are opening all the way.Is the bottom clean and smooth?

 

[MBAKER]

Re: confused on prop change

When you say the foil is killing performance you mean top end or overall? I really not concerned with top end speed. Right now im concerned about lugging the engine harder than need be. But I planned to try without it anyway, I kind of questioned whether it even helped at low speeds.

With the new 17P prop i beleive the speed was 39 (GPS) at 4200. Dotn know about the old one. Also, this has all been untrimmed on smooth water, if you trim it up it will pick up some At best trim the new prop would do 4800 at 44 mph. But i have been doing all testing untrimmed. Maybe thats wrong too.

Bottom is good, Just cleaned/polished and all that a week ago.

I havent looked to see what the drive position is full down. I think its down slighlty past parallel with the bottom of the hull. Should it be closer to parallel with bottom of the boat for general conditiions? Maybe im trying to push to much water all the time where trimming it up a little on takeoff would get the bow up out of the water more. This is first boat with trim capabilities so kind of learning here. Still have the old boat so I could compare full down position between the 2 if i needed to.

 

[steelespike]

Re: confused on prop change

You need to trim up to the best speed and rpm.Normally you would trim in for hole shot and trim out gradually to the best rpm and speed.Of course you might moderate trim for the conditions.The foil probably would help hole shot would depend on trim,brand of foil etc and if its in the water at speed its killing top end.
There are much better choices than a foil.
You should be concerned about top end because it carries over to your overall performance.The faster you can get from here to there at a given throttle setting
the better your fuel use.imo a 5.0 in a 21 should approach 50 mph.

 

[glengold]

Re: confused on prop change

What I've read (and done myself) when testing props is to trim up until the prop stops catching (cavitates) then trim back down right to the point it catches. This is where your lower end is creating the least amount of drag, and prop can spin the fastest. Sounds to me like you are right where you need to be, WOT at 4800 right? I have a dolphin fin on my outboard, its a 55hp pushing a heavy 16 ft fiberglass center console. This helps my boat get on a plane and stay on plane when trimmed all the way down. It also decreases rpms considerably when trimmed down. After i get on plane i can start trimming up to get more rpms and speed out of it. Many people will tell you with a dolphin fin on you can use a higher pitched prop because the fin will help the boat get on plane, once on plane the higher pitch prop will give you more speed. My fin is on the cavitation plate, which rides right at the waters surface, as it should. If you're cavitation plate is 6 inches underwater, something's not right....

 

[glengold]

Re: confused on prop change

I want to add if you adjust your prop so that you are turning 4800 WOT trimmed down, what will happen when you trim up? you may be turning 5200-5400 WOT, way above the manufactures recommended range. This would not be good for the life of you motor.....

 

[glengold]

Re: confused on prop change

whoops, forgot to mention about the full down trim, i needs to be past parrallel for the fin to work properly right? at this angle, the fin helps lift the stern out of the water and get her on plane. adversely, if it were trimmed up a little, it would dig the stern in, making it harder for her to get on plane. I think the 19 was fine, and a newer one would get you close to 50 mph. If you take off the fin, you might want to go down to the 17, if planing out seems to take awhile. You mentioned ski's, so in my case I have a 13 pitch for watersports and a 15 for max speed. Without the fin on my outboard, i couldn't plane w/ the 15 pitch prop and a heavy load!!

 

[MBAKER]

Re: confused on prop change

I was basing it on the mercruiser manual, stated that you should prop to the WOT range without excessive trimming. So I was trying to get into that range as is. Most of the time it seems like you cant really use the trim that much anyway because of conditions, but again kind of new to trim function. Yes it will get into that range if you trim up as much as possible. Im not worried if it ran past that at full trim because I can idle it back a little especially if lightly loaded and 99% of the time we are never full throttle anyway. Plus there is a rev limiter that will stop it and give you a warning to slow down without really getting carried away.

Our old boat, was an older bayliner, had a 3.0 OMC in it, no trim just ran it down all the time. It feels like it has better holeshot and it will rev to redline with no problem, you just have to hold it back a little. I know its under propped but worked well for towing and like I mentioned above we rarely ever run at full speed anyway. Seems like the water is usally rough enough you cant get up and run on top the water you almost have to leave the boat down in the water and ride over the chop to keep from beating it to pieces.

On the fin?
The fin is on the cav plate, but when I was watching it last weekend it looked like the tilt cylinders were right at the water surface, and the plate/fin is well below that that?s why I said 6-8? underwater even at speed. That?s at full down where it should be trying to pick the boat up but its obviously not. I guess I was somewhat curious as to whether they were a benefit on a sterndrive vs and OB where you can move stuff around a little more.