1, 2 OR 3 people: same top speed

[superwooter]

i have a 16' tri-hull boat with a '78 mercury 70hp thunderbolt (see pic below). no matter whether i have 1, 2 or 3 people in the boat, my top speed is always 35mph. someone suggested to me that it may mean i could stand to have a prop designed more for speed and that my current prop is putting out more torque than necessary. does this sound plausible? i am within the acceptable WOT RPM range and i get a terrific hole shot. is there any other information you need to assess this theory?

thanks for any input,

superwooter

 

[superwooter]

Re: 1, 2 OR 3 people: same top speed

sorry.
i just read the "read this first" thread. i'll get the pitch and diameter off the prop when i get home from work. i have no idea about the weight of the boat though. just let me know if there's any other info i can get to help with figuring this out.
thanks,
superwooter

 

[steelespike]

Re: 1, 2 OR 3 people: same top speed

Yup; get us the prop size (usually on the barrel or under the prop nut)
max rpm and speed preferably by gps.

 

[superwooter]

Re: 1, 2 OR 3 people: same top speed

took the boat out this past weekend and got the info you need. max rpm was a notch over 4900 (4800 - 5200 acceptable range), top speed on gps (and speedo) was 33 with 1, 2 or 3 people, time to plane is somewhere between 4 & 7 seconds, and i took a picture of the markings on the barrel of the prop....

so again, the proposed theory was that since i show no decrease is speed when adding a second or third person, could i stand to use a prop geared more toward top end? is my current torque a bit overkill?

 

[hwsiii]

Re: 1, 2 OR 3 people: same top speed

Wooter, I find that your motor has a recommended WOT range of 4,800 to 5,200 RPM and a Gear Ration of 2.3:1 and the picture shows a Mercury prop with a 13 pitch, please verify this.

If in fact, this is the case, then somewhere there is a major problem, because I show a prop slip of -26%, and that is impossible, so if my facts are right then you either have a tachometer is NOT right or a speedometer that is Not right. You need to take a GPS with you and a mechanics tach the next time you go and give us some better numbers.

Prop Slip

Please get some new numbers and get back with me and I will help as much as I can.



H

 

[superwooter]

Re: 1, 2 OR 3 people: same top speed

as far as verification goes, the prop is what it is and the speed, as stated was both gps and speedo. the WOT you have is what i have found for this motor as well. the numbers i can't verify at the moment are the 2.3 : 1 gear ratio and i don't believe i have access to a mechanics tach unless this it something i can borrow from autozone, o'reilly's, etc.

can you, in layman's terms, explain what you think might be happening?

thanks for the input,

superwooter

 

[trendsetter240]

Re: 1, 2 OR 3 people: same top speed

can you, in layman's terms, explain what you think might be happening?

Your prop is a 13" pitch prop. That means for each complete revolution it would move forward a maximum of 13".

The numbers you provide show the prop is moving forward 23% farther than 13" for each revolution which is not possible.

So either the Speed, RPM, Gear Ratio or pitch is not correct. If the speed is GPS then my guess would be the tach is off.

 

[hwsiii]

Re: 1, 2 OR 3 people: same top speed

Trend explained it very succinctly, as usual, that was a very good explanation.




H

 

[superwooter]

Re: 1, 2 OR 3 people: same top speed

i'm quoting this from another thread...

"My SELOC manual says, for 70hp 3cyl models 1977-79 and 1980-83, gear ratio is 1.64:1
(It says the 1987-89 70hp 3cyl is 2.3:1)"

i think that might correct the out-of-kilterness. would you mind running those numbers again, hwsiii?

thanks,

superwooter

 

[superwooter]

Re: 1, 2 OR 3 people: same top speed

i'm reasonably good with math so i just punched out the equation with the new gear ratio. i came up with 36.7816...mph in a perfect scenario which means 10.28% prop slip. is that good, bad or ugly? also how does that factor into my original question?

thanks,

superwooter

 

[QC]

Re: 1, 2 OR 3 people: same top speed

Good exercise, but it does nothing but tell you what speed you should be going if all else was right. Since the one number we trust is speed, it's not a whole lot of help

I'm with the others. Tach is wrong and needs to be verified before we can come to any reasonable conclusions.

 

[superwooter]

Re: 1, 2 OR 3 people: same top speed

Did you notice that I was saying the Seloc manual states a different gear ratio than what was originally assumed by hwsiii? If I understand correctly, the impossible prop slip indicates that SOMETHING is wrong; not necessarily the tach. Since the manual proved that the gear ratio was wrong, doesn't that negate the probability that the tach is off? 4900 is well within the WOT range.

 

[QC]

Re: 1, 2 OR 3 people: same top speed

Missed that, sorry. Actually your 33 MPH is like you said 10.28% slip. That would be damn near perfect. Zero slip is also fairly impossible. The only reason I say "fairly" is that there are some weird exceptions with some types of cup etc. But for this board, generally speaking, anything below around 8% slip is seriously suspect and 9 -15 should not cause much concern with the latter being heavier hulls and the former very light and efficient (flat).

I don't understand the no change in speed with different weights, but that hull looks pretty cool. Can you post a bigger picture? Maybe it's a lost super duper perfect design or sumpin' . . .

 

[superwooter]

Re: 1, 2 OR 3 people: same top speed

Sure can. I'll snap a few more shots this evening. I got to watch a friend drive it last weekend while standing on the shore. That was a first for me. I noticed that the hull sits up out of the water higher than I would've imagined. I'm sure that helps to reduce the drag.
Ok, that's enough texting while driving for me.
Superwooter

 

[superwooter]

Re: 1, 2 OR 3 people: same top speed

sorry for the late response. i've been out of the office on the road for a week and i do all my iboats posting from the office.

we went skiing/kneeboarding with the boat this past weekend. it was the first time in 10 years that i'd been skiing. i did some tweaking on the trim and was able to get the rpm to 5000 and the top speed to 34. there was a lot of chop on the water as it was very windy and there were about 15 jet skis zipping around at all times.

here's those pics...

but back to the original question. i've never done any messing around with props so i'm looking for some pointers here. should i try a 14p or 15p? what does the diameter play in the calculations? in the engraving on my prop (see picture in a previous post) there's a 48 at the beginning. is it correct to assume that this is my diameter in mm?

thanks

superwooter

 

[jrs_diesel]

Re: 1, 2 OR 3 people: same top speed

If your diameter was 48 mm, that would be an extremely tiny prop. 48mm = 4.8cm = 1.88 inches.

Now if it was 48cm, that would be a diameter of 18.89 inches, which strikes me as an oddball size.

IMO I don't think that 48 on your prop indicates diameter.

 

[QC]

Re: 1, 2 OR 3 people: same top speed

48 is just the beginning number for most Merc props . . . I don't know the significance, but does not seem indicative of anything important.

 

[superwooter]

Re: 1, 2 OR 3 people: same top speed

I see. I guess I have to just measure the diameter. When I select another prop, do I need to try to keep close to the same diameter or what.

 

[Fl_Richard]

Re: 1, 2 OR 3 people: same top speed

Just throwing this out there.... No picture with the motor down. I'm sure you've read many post here about how far up/down the engine should be mounted.

From the last picture it looks like it could be a tad deep.

 

[steelespike]

Re: 1, 2 OR 3 people: same top speed

48 Is simply part of the number To my experience Merc props begin with 48.
As far as diemeter it simply follows the design.If the prop is for your motor the diameter will be fine.On very rare occasions you sometimed have to change the
anode to clear the prop.
If you go to a 15" prop same brand same model you will lower your rpm "about"
400.If the motor can spin it up enough you should gain some speed and lose a little hole shot and load push.
Very unusual to not lose some performance when adding 300lbs (?)There is a slim possibility that with one aboard poorly trimmed that it might run better thus the same with one or 2 more because it trims better.(try to figure out that sentance)!