TMD, anyone , ? about WOT RPMS.

[KCLOST]

I posted a while back about my 86' Merc 175 that has a four bladed 24p prop....But didn't really get a concise answer.<br /><br />I'm pushing a Ranger 371V, 18 ft, 1988 boat. There is a Jack Plate....<br /><br />At WOT, does 5200-5400 rpms sound normal? The motor runs fine, but wanted to make sure I didn't need to make any throttle/carb adjustments to optimize.<br /><br />Thanks

 

[The Marine Doctor]

Re: TMD, anyone , ? about WOT RPMS.

If you go to the Mercury Marine site..they have a prop calc there. You can enter the weight of your boat and engine site etc...you can then find out if the correct prop is installed.<br /><br />TMD

 

[LubeDude]

Re: TMD, anyone , ? about WOT RPMS.

Picking out a prop is probably one of the most touchy things there is, espesially trying to help someone on a site like this! It can at times seem like rocket science! What makes one person happy will simply not work for another! I think it was walleyehead that took a half dozen props to the lake one day and posted all the results, he put in a lot of time experimenting with engine hight and different things, Im not sure that he has settled on one yet?? Maybe we can get him in on this??<br /><br />If you are happy with the performance of your boat as is, then you might as well leave it alone, If you are a little like I am you always think that there is more!<br /><br />Ater about 60 to 65 MPH, there are a lot of things to concider other than props, and it can give you white knuckles real guick!<br /><br />Is your Jack plate electric?<br />Can you trimm all the way out and not blow out or ventilate?<br /><br />You have a few hundred RPMs to play with, with your current prop!<br /><br />What speed are you getting now?<br /><br />Did you happen to catch the post, (Am I missing something), I beleive it was in the general boating section, a few weeks back? That shows you how things can get out of hand real quick!<br /><br />What do you want to acheive? Top speed, Hole shot, be specific and we will be all over it!<br /><br />LubeDude

 

[KCLOST]

Re: TMD, anyone , ? about WOT RPMS.

LubeDude, <br /><br />Really, I'm not interested in more speed... I can get the boat up to about 60mph.. But that's by trimming out to the point that the steering is really comprimised and the boat, quite frankly, is a little scary to drive...<br />But I've read that the normal RPM's at WOT for my motor is around 5800 or so...<br />So I just want to ask (taking into account my prop, boat, etc.) if my 5200 rpm reading is ok. If not, could there be a problem somewhere....<br /><br />I know it would be hard for anyone to answer this without a hell of lot of experience. Boats, props, weight, jack plate, shaft length, etc. all have an effect....(my jack plate is not electric btw).<br /><br />Anyway, Trimming all the way out? How far is all the way? I'm not willing to find out on my boat... I do have a trim gauge, and normally I don't go anywhere near the red, staying right in the middle of the white area of the gauge is my max.(how far out that is I can't tell you). It never has blown out or ventilate, but the steering does come into play like I mentioned. It gets a little squirrely! If there is a trim stop on my boat while running, I guess I haven't found it yet...<br />Hole shot is excellent.. I just want to make sure the motor is running at an acceptable RPM at Full throttle and a safe range of tilt out...

 

[Hasbeen]

Re: TMD, anyone , ? about WOT RPMS.

I beleive that your Max RPM for this engine is 5500-5800! It sounds like if you were more interested in flying than boating and if you trimmed a little more you would probably achive an RPM closer to this! It does put a little more strain on things buy not operating in the optimum RPM range at WOT, but I really doubt that you are hurting anything! If you dropped to a 22 Pitch, your boat would jump out of the hole, and sence you do not fully trim out it would probably put you in the 5500 RPM range! Personally I think you are ok, unless you need a spare prop and want to spend $350.00-$450.00 dollars!!<br />I actually have one you could borrow, but its quite a drive to come get it!! Id tell you Its for sale, but I understand that we are not to advertise things for sale on the forum!<br /><br />Hasbeen

 

[ronmold]

Re: TMD, anyone , ? about WOT RPMS.

If you could find a 22P 4 blade you would get about 5800 RPM w/your boat and hole shot would be even better. You should also get another 1 to 2 MPH @5800. Of course it will suck gas even faster (and wear out the motor faster). The steering problem is not from the speed but from the trim. There's only a couple feet of boat touching the water at high trim & high speed, no wonder the steering is eratic. Try raising the motor a notch or so (1") and check your speed, it should increase w/ less trim (and better control) but watch out for ventilation. You'll know when you get it too high. I have a manual jackplate also with an overhead hoist in the garage for adjusting quickly. Adding the jackplate made a huge difference in high speed steering control. The next step would be a dual steering cable setup & solid motor mounts. But really I don't need much more than 65.

 

[KCLOST]

Re: TMD, anyone , ? about WOT RPMS.

Thanks for the replies,<br /><br />I think I will try to raise the motor a little to see what happens.... One thing that has always bothered me is that the upper exhaust ports are almost completely under water with the motor tilted all the way down at idle or near no-wake speed. However, looking at the cavitation plate on the lower unit it looks like it is pretty much in line with the bottom of the hull... Is moving it up a risk? Whatever I do I'll be carefull, no doubt...