Need help from Ron Hill!!

[Nason]

After running it today my best numbers was 18pitch prop turning at 6200rpm and I only could get 34mph out of it. I don't know what the ratio is but its the small gearcase for the 2cyls

I got the same prop,except it was tried on a 40 Mercury,42 mph was the fastest it would go. Slapped on a 3 blade power tec 10-3/8 x 15p fishing prop 45.2, slapped on a 10-3/8 x 18p mercury chopper 49.3

[zul8tr]

After running it today my best numbers was 18pitch prop turning at 6200rpm and I only could get 34mph out of it. I don't know what the ratio is but its the small gearcase for the 2cyls

For the 18" pitch, 6200 rpm, 2.42 gear ratio, and 34 mph that is about 22% slip which is very excessive especially if a light rig, this indicates probably more pitch needed for your rig. Do you know the total weight of the rig running?

With the 22" Ron is sending if you keep the same rpm(?) and slip in the 8% range the expected speed is about 49 mph.

Although not the last word like testing is I find this reasonable for estimates:

http://continuouswave.com/cgi-bin/propcalc.pl

[Ron Hill]

For the 18" pitch, 6200 rpm, 2.42 gear ratio, and 34 mph that is about 22% slip which is very excessive especially if a light rig, this indicates probably more pitch needed for your rig. Do you know the total weight of the rig running?

With the 22" Ron is sending if you keep the same rpm(?) and slip in the 8% range the expected speed is about 49 mph.

Although not the last word like testing is I find this reasonable for estimates:

http://continuouswave.com/cgi-bin/propcalc.pl

That prop chart is excellent, I just forget about it most of the time.

zul8tr, what peed to you think this boat should go? I can't quite see it. I know you've run a bunch of these boats.

Is this a 2 cylinder 50 or is this the three cylinder? I just don't recall this gearcases water pick ups.

[omcstratos]

Where is the center of the prop shaft relative to the bottom/pad of the boat?

What kinda boat and how much does it weigh?

I have tried everything from the prop submerged (and get slip) to running as high as water pressure will allow (not much better performance). It seems like it runs a little better up high even to plane off. Boat is a 16ft modified vee bottom even though looking at the back it looks more like a flat bottom to me. Its kind of heavy being 350lbs.

That prop chart is excellent, I just forget about it most of the time.

zul8tr, what peed to you think this boat should go? I can't quite see it. I know you've run a bunch of these boats.

Is this a 2 cylinder 50 or is this the three cylinder? I just don't recall this gearcases water pick ups.

Mine is a 2 cylinder 50 and the last of the OMC's made had the plastic water pickups before they became BRP. I wish it was a 3cyl,, I like the sound of them better haha!!

[zul8tr]

That prop chart is excellent, I just forget about it most of the time.

zul8tr, what peed to you think this boat should go? I can't quite see it. I know you've run a bunch of these boats.

Is this a 2 cylinder 50 or is this the three cylinder? I just don't recall this gearcases water pick ups.

Ron

Thanks Ron I always try to post helpfull stuff. It is somewhat hard to say what speed the boat should go because I do not have real good info on his boat. I would like to see the bottom, if there is a pad, a hook or rocker, the total weight when running, etc. But since I can't tell what his rpm's will be with say the 22" prop (you are sending) compared to his posted 6200 rpm with the 18" pitch all that can be done is estimate with the formula I posted, so my present estimate is near 50mph with the 22" if the 6200 can be sustained with low slip (8%). I have used the calculation with a lot of the boats I have run and in the light hydros I get very near the predicted speed with about 4% slip. I usually calc slip for a setup with the formula by running a measured distance and timing the distance covered that gives avg speed then use the formula to get slip. Then slip can be used in other calcs with that prop and rpm. The heavier the hull and depending on the bottom design the more slip that will occur for a planing hull. Props have always been an experiment to get them right. I suspect he will have to lift that 22" pitch baby way up to get the revs. Hope his water pickup is low enough? From tje pics posted of the lower unit I suspect it can be raised at least as high as so the bottom is level with the low horizontal crease on the water pickup slot because water will rise up the case from the bullet nose and get in the slot. Note that when hull weight increases the operation of fully airated props (surface piercing) on vee bottom hulls becomes more difficult to achieve with low slip and to keep the revs up with bite.

Pete

[omcstratos]

Here's some pics of the boat for you zul8tr. the boat is true all the way down but it seems to have a little bit of hook 6in from the transom maybe of 1/8 of an inch. I also have what I think you call an "ice runner" down the middle of the boat, I wonder if it creates any type of a disturbance in front of the prop. I measured along the width of the boat and starting about 3 feet from the transom, the boat becomes a flat bottom with no vee at all. Also my yard stick was the longest straight edge I had and checked it with a laser, it is truly straight. Well here ya go...

Attachment 54766Attachment 54767Attachment 54768Attachment 54769Attachment 54770

[zul8tr]

Here's some pics of the boat for you zul8tr. the boat is true all the way down but it seems to have a little bit of hook 6in from the transom maybe of 1/8 of an inch. I also have what I think you call an "ice runner" down the middle of the boat, I wonder if it creates any type of a disturbance in front of the prop. I measured along the width of the boat and starting about 3 feet from the transom, the boat becomes a flat bottom with no vee at all. Also my yard stick was the longest straight edge I had and checked it with a laser, it is truly straight. Well here ya go...

Attachment 54766Attachment 54767Attachment 54768Attachment 54769Attachment 54770

The attachments do not open?

[omcstratos]

Here's the pics, refer to my above post for the text. I've edited this one too much to get the pics right

[Master Oil Racing Team]

That riveted V will cause an air tunnel. I have seen it happen with an SK fin mounted several feet in front of a Merc speedmaster lower unit. The front of the SK fin was sharp, but the backside was totally flat. 90 degree flat. So that let an air slip develop. The faster we went the further back the air hole slipped until at 70 mph the air hole connected with the water intake holes. This speed higher than what you are experiencing, but that open holed "V" runner in front of your prop will disturb the water. It might not make any difference, but you could mix up some good epoxy....shove it up into that vee, and hammer it closed from both sides. Make the trailing edge come back together. We solved our problem by grinding the back side of the SK fin so it was sharp also. That made the water close back in behind the fin. That fin was not made for what we were doing, but after experimentation it made all the difference, and that's what testing is all about.

[zul8tr]

Good tips from Master Oil Racing Team, do them. The hull is a flat bottom with runners on the bottom that definately will affect speed and water feed to the prop and were placed there to stiffen the bottom, usually they put those on the inside but maybe they wanted extra protection on the bottom for beaching, etc? Plus the hook is just enough to push the bow down at speed and removing it would help. These small alum jon boats do not really have speed designed into them so you definately have a challange to get to 50 with that engine. Because this hull has very little aero lift and some very small amount of air compression under the hull to help lift all your speed gains will be related to reducing water drag which is far greater (about 780 times more that air drag if density is the only varible changes). You can reduce some water drag with a higher engine height if you can keep the prop pushing, but hull drag is another matter that will come about by lifting the bow so the hook removal will help here and getting rid of depressions and bumps in the last 4 to 5 feet of the bottom. Hook can be removed with epoxy thickened with cabisil (fumated silica) and leveled on to the correctly prepped alum bottom. But before doing this do the Master Oil items then fool with the next prop Ron is sending then clean up the hook and bottom irregularities still be prepared to fool with other props.