Advise on prop/RPM stats

[Nandy]

I finaly got a hold of a tach and check my stats today. This is what I have:

1975 Cobia Tradewinds SS-170
Close Bow 17 Footer V-Hull
1975 Evinrude 70573-B 70HP

These are the numbers with a "close to" typical load (just 2 riders instead of 4)

13 1/4" dia x 17p aluminum prop
Idle RPM under load 650
Idle RPM off load 1050
WOT RPM (30 MPH) 5450
"Cruising" RPM (20 MPH) 3950

Speeds were taken by means of a GPS. Also, with the typical load the boat dont seems to loose any speed BUT, not sure if the RPM will not change but I would guess not much.

The main use for the boat has shift from fishing to cruising on the lake and the occasional pull of a tube. I have a feeling that with this new to me rig the tubing activity will increase. So, what I am looking is a prop for cruising and one for tubing.

Can any of you provide any advise or impressions on the numbers? I have not weighted the rig yet, that is on the works but it will need some logistics as the lake and dumpster are quite far apart. I will try to get a photo this weekend of the way the motor seats on the transom as I was requested on my initial post http://forums.iboats.com/showthread.php?t=262533

As usual, thanks for the help!

 

[steelespike]

Re: Advise on prop/RPM stats

I'm sure no rpm expert but as I understand it the 75 70 hp is rated at 5,000
it seems to me running at 6,000 may be stretching things a little.
I'm sure someone in the know will let us know.
If your running now with no venting problem you might try raising the motor a hole or so.You could pick up a little rpm and speed.

 

[Nandy]

Re: Advise on prop/RPM stats

I was told 5500-5900 RPM. I havent seen my book but I take it as good source. I might do some sync/link tomorrow or sunday. I will have my book by then.

 

[steelespike]

Re: Advise on prop/RPM stats

I was told 5500-5900 RPM. I havent seen my book but I take it as good source. I might do some sync/link tomorrow or sunday. I will have my book by then.

Thats probably right.Its just that my aftermarket ser. manual has a sub note 70hp motors prior to 86 are rated at 5,000.
NADA doesn't list a Cobia tradewinds only a cutless at 1300 lbs and riviera? at 900 both pretty beefy for a 70 hp.
Considering your pretty generic setup I don't think 30 is too bad if the boat is 1300 lbs.And I think the 17" prop is close.A 15 would put you at about 5,850 Do you have a reason
to suspect the lync and sync?
Unless allready done I think I would spend some time fine tuning the setup.before messing with lync and sync.
could give you a couple of mph and some rpm under ideal conditions.

 

[Nandy]

Re: Advise on prop/RPM stats

I also have a Seloc manual and it shows it to be 5k rpm like you say, also the idle rpm is 700-750. Anyone has an OEM manual that can clear it this up? Is a 1975 Evinrude 70573-B 70HP like my signature indicates. I hate to be ruining this motor by running it wot and being almost 500 rmp over it.

I took a few photos of where the motor seats in the transom and the plate seems to be an inch lower than the hull. I think it is ok but I surely have space to raise it as the motor bracket is at the top most hole. That is not something I will do today as I have a trip tomorrow with the boat, but will try if the photos reveal it is worth a shot. Here they are:

Here I am using a piece of wood to make it easier to see the motor in reference of the bottom of the hull. This wood measures 1 1/2 inches by 3/4 inches. Here is with the 1 1/2 height.

3/4 height.

Here you can make my water line.

As you can see the motor is at the top hole so I can raise it quite a bit.

 

[Nandy]

Re: Advise on prop/RPM stats

Anyone?