What is the right R.P.M.

Theron · 11-11-2006, 10:03 PM

I have a 28' glasply with twin 350 chevs. and v drives could anyone tell me what the max R.P.M. should be and what whould be the best full speed cruising R.P.M.

Thanks Theron

MattPark · 11-11-2006, 10:09 PM

Every engine setup is different, but I would think 4500-5000 is probably the ballpark.

Theron · 11-12-2006, 08:26 AM

Thanks the guy I bought the boat from said 32oo and a car mech. said that was right and then some boat guys said the same thing you did and the boat doesnt even set good until around 4000 I just didnt want to run her that hard if it was too much. Thanks again

Theron

MattPark · 11-12-2006, 11:25 AM

My guess is it will probably plane at 3200 depending on what props you are running, and the slowest planing speed is usually close the most efficient speed with a larger fiberglass boat. Maybe that's what he was talking about.

You definitely don't want it at 3200 at WOT.

ron m · 11-12-2006, 02:03 PM

If you don't have a fuel flow meter, installing one might be a good idea. I think some folks have it hooked to their gps unit and it'll give them the miles per gallon. If it's not that fancy, you can divide the speed in mph by the gph and you'll get the mpg. The m can be either statute miles or nautical miles.

With my boat/motor/prop the most efficient speed isn't just above planing speed, it's quite a bit faster than that. Like 20 knots or more and the boat planes at about 12 knots. Actually that's not quite correct, about 4 to 6 knots is way more efficient, but a bit slow for traveling any distance.

luckyr · 11-12-2006, 06:03 PM

My 78 Searay had a 350 chevy and 5200 rpm was the absolute top.

Theron · 11-12-2006, 06:56 PM

Thanks for the input

yes I do have a flow meter but let me tell you the story leading to the question.

When I bought the boat it was on the water in portland I floated it to kalama marina and never had it out of the water (didnt have a trailer with it) So at that time wide open the most rpm I could get was 3200-3300 rpm and that just hapend to be the rpm the saleman told me to run it I thought it had a govenor on it and the best speed I could get was about 25 mph and getting about 1 to 1 on fuel eco. and that was working for it this beeing my first larger boat I didnt give it a second thought, but I was a bit disapointed. Then later in the summer I made a big rookie boo boo and while headed for tune fishing I ran her onto a sand bar in the columbia some of you may remember the post "looking for tuna found Oregon"
anyway in that incedent I bent the port drive shaft and pretty much destroyed the props so I put it on a rented trailer and put in a new shaft and had both props rebuilt and put it back on the water.
Heres were my questions started after doing the work I went out for test drive and it is a completely different boat, I can now hit 4000 rpm with ease and 30 + miles an hour wasnt even a problem but that was were I stopped until I talked to somebody because I didnt want to hurt her.
So pretty much I have sermized that the props were messed up when I bought the boat and the salesman was incorrect I had no Idea that a prop could make such a drastic diferance in rpm capabuility.

Now that I have the confedence to do so. I cant wait to get back out and open her up and see what she will do.

Thanks again for the help

Theron

P.S.Like I said this is a 28' glasply, about a 12000 lb boat what should I expect for fuel eco.

Chrome Bumper · 11-12-2006, 08:04 PM

1 sounds about right. You could do better at 6 knots.

Pilar · 11-13-2006, 04:29 AM

I run one chev 350 but with an outdrive not a vee and the 26 foot boat weighs 7000.

The boat planes at about 15kts, 3000rpms. Runs pretty good at 20kts, 3300 rpms and maxes out at between 28kts and 35kts, depending on what's on board. The 350 chev redlines 4200 to 4600, according to the Seloc repair manual.

I'm pretty sure I could change the prop since I never get above 3900 fully loaded and never above 4200 empty. Most base the prop performance on redline rpm. I just used the props I bought with the boat. They have better if you have the money.

Load is a big factor and so is balance.

Theron · 11-13-2006, 06:18 AM

The props on my boat are still the ones that came with it but I am pretty sure they are going to let her max at just about the right rpm. The rpm"s and speed I mentioned are kinda guessing on memory I am going to take her out today and run her out to see what she will do Ill write on here this afternoon and put the exact numbers.

You mentioned ballance . I realise that prop ballance is criticle but what about the boat its self? My boat lists pretty good to the port side while moored and no one aboard and sets what apears to be heavy to the rear (I now the back IS heavier

) My comment is however made based on the water line painted on the hule at the back with no one onboard it sets just below the water and the front is way above the water maybe thats the way its supposed to be

I will pay attention to the other boats when I go there today

Is there anything else I should watch for?

Thanks again Theron

Metal Manipulator · 11-13-2006, 06:33 AM

I was told by a boat dealer that when you start from an idle speed and then push the throttle down to WOT that the RPM should go to 3800 to 4000 RPM. When I upgraded my boat with all the fishing goodies and extra fuel I could just barley get her on plane. I had to do a pitch change on my Bronze prop. I asked questions and was told that a 1 inch. pitch change would yeald aprox. 200 RPM change so I went from 10 inch. pitch to 9 inch. pitch, which was as tight as the propeller buisness said that I could go.

When I put the boat back in the water the RPM's went from 2800 to 3400 on the hole shot and would top out at 4000 not the best but with the 307 (200 hp.) I figure that would be the best It would do. I now can get up on plane fully loaded, 500 lbs. of Ice and 3 on board.

The motor does not have enough Hp. for what I am doing so I will have to up grade sometime in the future. I am hoping to aquire a 350 EFI with 325 hp.. The boat probably will not burn any more fuel than I do right now and poossibly a little less because I have to run nearly WOT when loaded to travel at 20 - 25 mph. (sorry I haven't converted yet, which will happen with the new fuel flow meter). with the addition of trim tabs the boat planes at 14 mph. on flat water. This slow speed can not be maintained with Ocean swells. I would like to be able to maintain 20 to 22 mph. with this boat on following seas having 4 to 5 ft. swells and fair intervals. I am not into beating up my equipment so I will not run at speeds which produces severe pounding on the hull. Fiberglass is a very good material but even it has its limits.

Pilar · 11-13-2006, 07:44 AM

I guess I was not specific enough. By balance I am talking about list (lean to the left or right) and trim (bow down or bow up). Although your trim tabs will correct this while you are running they cannot correct too much weight in front or behind the center of pitch. And if you run with a following sea trim tabs severely compromise control of your boat.

You can counterbalance by placing weight on the opposite side of the center of pitch but you set up a pendulum by doing so.

There are several of us on this forum with heavy glass boats that know all too well about list and trim. On my boat to fish for TUNA! I have to get the ice in the fish pit which is just aft of the center of pitch.

If I put ice in coolers on deck and it slides aft the bow comes up. Even with the weight down low it is hard to plane out with the crew in the cockpit. I have to send them forward if we are going to run for any distance at cruising speed.

Metal Manipulator · 11-13-2006, 08:38 AM

John is right on this, as I have to do the same. Trimtabs are a great help but you should set your boat up to minimize their use. I have not experianced the following seas problem but I have been warned by many here on this board ,so I do bring my tabs up to avoid finding out the hard way.

Chrome Bumper · 11-13-2006, 08:45 AM

Two more feet of boat immediately aft of the existing transom will provide bouyancy aft.

fyshndad · 11-13-2006, 01:30 PM

Quote:

Originally Posted by Chrome Bumper

Two more feet of boat immediately aft of the existing transom will provide bouyancy aft.

Steve I think you meant 20 feet didn't you????

Theron · 11-13-2006, 06:34 PM

OK so if I get this right what I need to do is add 60lbs of lead to the starboard side and bolt the livingston trasom to transom with the boat

O K so I took her out today three men on board max rpm 3900
max speed 33mph
Fuel 30 gph
I couldnt break 4000 rpm but I could get to 4000 rpm with a good deal of ease the fuel eco didnt come up but I dont know how much faith I have in the flow meter as it is newly installed I will check it the old school way next time I take a trip . I would liike to have the other 500 rpm but I am a lot happier with her now than I was

Theron

Metal Manipulator · 11-14-2006, 06:07 AM

Good for you!

Props and Trim tabs make for a better boat if set up right.

Now it will be a long winter waiting to go bloody the decks.

Theron · 11-14-2006, 06:40 AM

Yaa I know I just got a good taste of the blood last year then got shut down and ran out of time would like maybe to fish a tourny this summer???

Thanks again for the wisdom

Theron