I think my boat is slow, but Im not sure why. Any ideas?

[rwoodruff]

If I am posting in the wrong spot then let me appoligise up front guys.

I have an 81 All family craft charger 1650 open bow that thanks to help and advice from here I have been able to rehab and take back to the water.

It is a 16 foot, 6' beem with a mercruiser 140hp 3.0L. I have a 17 pitch by 15 prop on it now, and I also have a 21 pitch x 14 1/4. the boat weighs 1900lbs and we usualy run 3 adults and limited gear.

We have had it out a few times now and seems to be reliable. engine powers good, sounds good, starts fine.

Now the performance question. for this boat size and power I seem to only run about 20 to 25 mph at WOT. RPMS are good at 4500. whole shot is ok, it just seems slow to me.
When I had the 21 pitch prop on it I did get about 5 to 10 mph more, but the whole sucked.

What am I missing here? Is there something I can check here that is missing? The fuel is new, the carbs have been cleaned and rebuilt. Fuel economy is good. I would just expect it to at least do 35mph.

I would realy like to here everyones opinion or sugestions on what to try.

Thanks,
Robert
St. Louis

 

[Pete104]

Re: I think my boat is slow, but Im not sure why. Any ideas?

Are you sure you are running the 1.98:1 gear ratio? Mercury used to have a small diameter (14') X 19"P. Still might, I didn't research all that much. I did find a 4 blade, 14 1/2 X 18. Contact a dealer w/ #QA2184X & tell them what have & they will set you up with a hub kit.

 

[thumpar]

Re: I think my boat is slow, but Im not sure why. Any ideas?

Something is wrong. I have a 16.5' sunrunner that sounds very close to your boat size but I have a merc 120. I hit 35 mph (GPS) with a 19 pitch prop at about 4450 rpm with 2 adults and 2 kids.

 

[LilRedNeckGirl]

Re: I think my boat is slow, but Im not sure why. Any ideas?

boat speed is fickle. first, the 140 horse/ 181 cid/ 3.0 typicaly has a 14.5 prop. mine does as well as every one ive seen. boat speed is drasticly affected by so many factors. First, never forget current, wave action and wind. 3 uncontrolable factors.
then factor in the trim of the boat, is it on plane with proper trim? look at marine growth on the hull, is the flotation soaked and extra heavy, ( one gallon of water weighs around 8 pounds) , then , on the transom, whats hanging there, the more gizmos & gadgets, depth finders, drive showers, the more drag you create, the slower you go.
just a wee bit more. i run twin 140 mercruisers. looking recently at new props, nearly every prop mfg. told me that anything over a 17 pitch is to much for the 140. you may get the top end extra 5 mph, but, its just way to slow off the line, and it will restrict your overall rpm limit. most suggested anything from a 12 too 14 pitch for all around running.

 

[rwoodruff]

Re: I think my boat is slow, but Im not sure why. Any ideas?

thank you for everyones responses.

the boat hull is clean there is no foreign debrit on it. there isnt and depthfinder or any junk on the back of it. I cannot find anything to cause it drag.

I am thinking I will have to play with the pitch, but usualy you are only talking about getting another 5mph there. The water conditions will slow it some, but under perfect conditions it still wont go anyfaster. I am not sure why.

 

[UNSUREBOATGUY]

Re: I think my boat is slow, but Im not sure why. Any ideas?

I have an 87 Four Winns 160 Freedom (16') with the same setup. Pretty much the same stock weight, load, etc. We run a 19 degree 14 1/4" prop which works great for towing the tube and such. With that prop, we see about 38 mph at 4500 rpm by gps and the throttle still has a way to go. I would suspect that something on your setup is off-speedo, tach or your weight. If the speed is by gps and the tach is accurate, you might want to see if you can visit a truck scale and check out the weight. Not sure about yours, but our single Axle steel trailer is supposed to run around 900 lbs. or so-just so you have a ballpark. I hope the foam isn't waterlogged. Good luck.

 

[Bonanzadave]

Re: I think my boat is slow, but Im not sure why. Any ideas?

With that prop, we see about 38 mph at 4500 rpm by gps and the throttle still has a way to go.

For comparison my 97 SeaRay with a 3.0L does 43mph on the gauge with just me WOT and trimmed up.

 

[TheBlizzard]

Re: I think my boat is slow, but Im not sure why. Any ideas?

Yeah you should be a bit faster than that, are you trimmed up at all, because you could be just plowing water which will slow you down drastically. I would think you would be able to do at least 30-35 with that.