Prop size vs. Gear ratio

[eddiej82]

Hi , new guy with a new to me boat. I have a 28' 1984 Chaparral 278 XLC cruiser. It has a single Mercruiser 260 with an Alpha one drive. The drive was originally a 1.50 ratio I think. I can't be for sure because all of it is painted over but all the info I've found says that is what it was. I'm not sure what prop this boat came with originally either but it had a 16" diameter 13 pitch prop.

Anyway the drive was bad so I replaced it with a 1.84 that I got from a friend that had a smaller diameter ( not sure what diameter) 21 pitch prop. I tried it with the 21 pitch prop and maxxed out at 4000 rpms and 20 mph according to the speedometer. I switched it out for the 16" diameter 13 pitch prop and maxxed out at 4500 rpm's and still 20 mph according to the speedometer.

I'm not sure if I was getting that much slip with smaller diameter prop or if the speedo' is off. I haven't tried it with the gps yet. I know this is a large boat but does only 20mph sound right? Anyone know how fast these were stock? Do I need a diffrent pitch- big diameter prop? Anybody have any ideas?

Thanks,
Eddie

 

[coolguy147]

Re: Prop size vs. Gear ratio

dude one word gps!! it will get u exact speed on the water speedoes r not accurate.


another thought also. its 28 feet!

 

[hwsiii]

Re: Prop size vs. Gear ratio

Eddie, the slip on the 16 x 13" prop is not real, you are showing a 34% slip factor at 20 MPH. If you have a lot of extra weight from foam that is filled with water, Barnacles all over the bottom or you are wrong about the gear ratio, that might explain it . But my assumption is your speedo is wayyy off, because you showed the same speed with both props. Run it with a GPS and give us the real number. And that boat with a 260 HP engine ought to go faster that 30m MPH, by my calculations. My findings show the boat weighing 4,500 #, I added 100 gallons of gas at 620# and two adults and two kids for a total off 440#, plus other standard supplies for a Total ALLUP weight of 5,800#. Here are my numbers on slip.

H

 

[eddiej82]

Re: Prop size vs. Gear ratio

I know that there is no waterlogged foam because I had to replace a section of plywood flooring in the cabin and everything under it was ok.

No barnicles as this is a fresh water boat.

I'm sure about the gear ratio of 1.84 because the drive I put on it looked brand new and according to my friend has never been opened before.

I'm going to put my Garmin in it this weekend to see what is going on but to be honest I couldn't tell the difference between the 2 props. They both "felt" like they were going the same speed at WOT.

Also they unit I put on it has one of those whaletail/hydrofoil things on it. Does it need to come off? Would it make a difference?

 

[hwsiii]

Re: Prop size vs. Gear ratio

Definitely take it off.

H

 

[hwsiii]

Re: Prop size vs. Gear ratio

Sorry, I looked up the wrong hull, yours has a 20 degree deadrise, and it will not do even the 30 MPH I said. You will want a prop with NO rake, as you need to raise the stern UP and out of the water. I would advise you to install trim tabs as well. With that little horsepower you need everything you can to pick the hull up and get it on top of the water. Get the GPS numbers and lets see what we might can do. At this stage I know it is using a lot of gas, because of being underpowered. But Let's see.

H

 

[eddiej82]

Re: Prop size vs. Gear ratio

The boat has trim tabs and they do work

The whale tail was on the boat that I pulled the out drive off of. It was on a 20' ski boat, not this one originally. I will defiantly try it without it on this weekend.

The original out drive that I removed had a 1.50 gear and a 13 pitch prop. The drive that I replaced it with has a 1.84 gear. What pitch prop should I theoretically run on the 1.84 to be in the range of the 1.50 with a 13?

The motor is the one that came with the boat and was the largest single engine option. They ranged from a 198 to a 260 and I have the 260.

Here is a pic of the boat from a brochure in 1984. I might be wrong but it looks to be going faster than 20 mph.

 

[hwsiii]

Re: Prop size vs. Gear ratio

It does look like it is going faster than 20 MPH, but my database says they also had the option of twin 170 horsepower, which would make the horsepower 340 total. The change would be from 1.5 to 1.84 gear ratio and from 13" to 16" pitch with the new gear ratio. But if you try to put a 16" pitch on it, the boat will turn less RPM and right at the same speed. Let's get the GPS numbers and then we will have a more Realistic picture of speed to work with and I have an idea of what might possibly work. To the best of my knowledge I don't believe you can put a larger diameter prop on that drive, and I think I have a better possibility.

H

 

[gpz5502000]

Re: Prop size vs. Gear ratio

I have an 87 278XLC with 260 Merc Alpha 1. Outdrive is a 1.50:1. You should be using different prop. 278 XLC's with single outdrive came with SS 14" x 19p. (Merc Crusier p/n 48-73980A4). I am using an alum. 14 1/2" 17p with some very generous cupping and my top speed is 39 mph @ 4400 rpm with a pretty standard load. I even use an alum. 15 1/4" x 15p cupped when I have heay loads and am still able to hit 36mph at 4600rpm