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View Full Version : Lower unit size/gear ratio/prop relationship



David_L6
12-17-2002, 10:57 PM
I know I'll have to test, but THEORETICALLY... if I switch from a Merc D unit (2.25 dia.) to a Yamato hatchet unit (I think its ~ 1.8" dia.) and gear ratio stays the same (1:1) what changes in ideal, theoretical prop size will that make?

Also, if I change from the above mentioned Merc 1:1 unit to the smaller dia. Yamato unit with a 14:15 gear ratio, what changes in ideal, theoretical prop size will that make?

Let's say I'm swinging a 7 X 13 at 8000 on a 1:1 Merc unit now, what size prop will I need if I change to the Yamato units mentioned above?

Ron Hill
12-19-2002, 12:35 AM
With the smaller (1.8) unit, you will get more water to the prop, so don't worry...use the same prop.

I always ran the same prop on 1:1 and 14:15, I got around the course so much faster with the 14:15 that I never got to top end anyway.

For one reason, I always used 14:15 over 1:1 I never went to a Boat Race to RACE, I went to WIN!...I know things change, but we'd test with a 1:1, then switch to a 14:15 for the race, (1:1 were stronger) but the 14:15 won races....

Exit speed at the pin is important, not entrance speed... On a record course you will average the speed at the exit pin...no matter how fast you go down the shoots!!! Keep that in mind when testing!!

David_L6
12-19-2002, 11:42 PM
Originally posted by Ron Hill
With the smaller (1.8) unit, you will get more water to the prop, so don't worry...use the same prop.

I always ran the same prop on 1:1 and 14:15, I got around the course so much faster with the 14:15 that I never got to top end anyway.

I never went to a Boat Race to RACE, I went to WIN!...

I like what you're saying! I WAS concened that switching units would cause a need for different props. I'm very glad to hear it won't.

Getting around the course quicker is what it's all about!

Well, the last couple of years I've been RACING. That's the reason for buying the Yamato units. I prefer winning over racing.

I did talk to one guy that tried a Yamato unit on his C hydro and he said he was faster with a Merc unit. I didn't think to ask him what ratio the Yamato unit was; he runs a 16:21 Merc. He thought the built in lift of the Merc unit was what made it faster on his C.

I bought two Yamato units with the intention of running the 1:1 on my E (APBA D mod - 44) and the 14:15 on my C (I currently run a 1:1 Merc). It appears that I need another 14:15...
I guess I'll just test both ratios on both boats and see what happens. I may also think that the Merc units lift is need on the C.
(BTW, the Cs that I am referring to are both Mercury Mark 30H C mods, NOT Yamatos! There are still a few stubborn folks out there that continue to run the Mercs and I am one of them.)

Thanks for the info, Ron.

David

Mark75H
12-20-2002, 12:23 AM
David, the "built in lift" is a fantasy. Quickies have the tipped up nose because Carl thought units like the Konig and Eldredge looked funny with the bulge in the gear area. Carl rejected early prototypes of the Quicky because of looks. It's cosmetic only.

The 3º tip up of the nose is not enough angle, nor does the front of the bullet present enough area to significantly produce any "lift"

We might have to bring Jimboat in on this if you need to see the math.

Ron Hill
12-20-2002, 10:15 AM
Sam:

I always thought that there was a little built in "Kick out"....."out trim" in a Quickie. The "D" Quickie handles worse, in my opinion, than a Yamato, Konig, or Eldridge...but, skeg length and general hand contouring could be the reason.

We always milled a "D" foot parallel to the cavitation line, two passes of the mill on the bottom, before we filled it...WE felt this made the waterline closer to parallel to the propshaft... Am I making sense??? (The bottom of a "D" Quickie was always a "Rocker" to us, another boat bottom to be straight...

The bottom of a Konig, Yamato, Eldridge was flat or parallel to the propshaft... At least the Hill Family thought that is why these units handled better in racing conditions..

A "rocker" will lift the bow, but I don't like rocker bottoms for circle racing..

Mark75H
12-20-2002, 02:54 PM
Ron, you are correct, you cannot base your set up angle on the Quicky case. Your only true angle is the shaft, which differs from the case by about 3º

I think you are correct about the skeg size being a possible difference. Look at the evolution of the Quickies: with each step the skeg became bigger with more area.

I have to give Marshall Eldredge a lot of credit on this. His lower unit had a much larger skeg than anyone else's for about 10 years.

David_L6
12-20-2002, 07:22 PM
I'm learning more about lower units here in just a few days than I have in 20 years of racing...

Sam, your statement about not enough angle to produce any lift has me really wondering about what gear ratio McAfee tried on his C. He said his Merc unit was faster. I do know that he runs 16:21 Merc and since getting a B&H like I have has cut the sides of the unit. Whatever ratio that Yamato unit was must not have been a good match for the prop. He likes and runs the Yamato on his E.

Ron, better handling is the main reason I bought Yamato units. EVERYONE that I know that has tried a Yamato unit on a D or E has switched claiming a world of difference in handling. My E unit (55H unit) has had the sides flattened, but nothing was done to the bottom. After reading what you said about the 14:15 gear ratio I doubt that I'll fool with the Merc unit much, but I may work the bottom of it and carry it with me in case I shuck a set of gears in the Yamato unit.

I appreciate the information given here, guys.