Modified RAGE 15 to Outboard

Repair or modification of Boston Whaler boats, their engines, trailers, and gear
absolutx
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Joined: Mon Jul 12, 2021 10:48 pm

Modified RAGE 15 to Outboard

Postby absolutx » Mon Jul 12, 2021 11:22 pm

I had a 1994 RAGE 15 when I was a teenager in 2000. In 2020 because of the pandemic I moved back home to where I grew up. I found my RAGE 15 abandoned (by me) in an old garage.

The hull was in great shape. I completely stripped out the rest. I read about other conversions here on continuoswave, and I decided on my own transformation for the boat.

I sealed the hull bottom and transom with fiberglass. I installed a stern bracket and a new Mercury 75-HP outboard engine with BayStar hydraulic steering.

My first test drive was yesterday, July 11, 2020. When the boat begins to plane at 3,500-RPM, the bow oscillates up and down badly. I headed home feeling like I just wasted a lot of time and money—hopefully not,

The modified RAGE 15 looks stern heavy, and the engine also looks a little low.

My first idea to do this week is to raise the engine mounting height a bit higher—who knows if it’ll work?

Give me ideas to try out.

This boat is intended to cruise around a bay with my family: my wife, my two beautiful girls ages two and five, and Boston Terrier.

I hope the conversion to outboard power works out. I put in a lot of money and sweat.

RAGE15.jpeg
Fig. 1. The modified RAGE 15 seen from the port stern quarter.
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RAGE15-2.jpeg
Fig. 2. The anti ventilation plate is about 1-inch below the lowest part of the hull—that’s not good.
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RAGE15-3.jpeg
Fig. 3. The remaining vertical distance the engine could be raised.
RAGE15-3.jpeg (82.48 KiB) Viewed 4707 times

jimh
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Re: Modified RAGE 15 to Outboard

Postby jimh » Tue Jul 13, 2021 8:49 am

When you attached the new stern bracket to the hull of the RAGE boat, its vertical position on the transom created the nominal transom height. The result looks like about a 15-inch transom height.

By mounting an engine with a 20-inch shaft onto the stern bracket, the result is the engine is mounted too low—maybe as much as 5-inches too low.

The stern bracket you have used appears to have a buoyancy box, but because the bottom of the stern bracket will never get out of the water, the buoyancy box is adding drag. Also, in Figure 3 there appears to be a flange at the aft end of the bracket that is getting dragged through the water, and that is creating more drag.

I suspect that changing the vertical position of the attachment of the stern bracket to the transom is going to be very difficult. Then the only option will be to raise the engine as high as you can with the five-mounting-hole sets.

jimh
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Re: Modified RAGE 15 to Outboard

Postby jimh » Tue Jul 13, 2021 8:56 am

The stern bracket shown above on the RAGE 15 hull reminds me of a Whaler Drive, a similar stern bracket with a buoyancy box and outer platforms.

Below is an illustration from my article on Whaler Drives that shows the vertical and horizontal distances used by Boston Whaler when mounting a Whaler Drive onto an OUTRAGE hull. I am sure that in developing the installation height there was a prototype hull and some experimentation was necessary to refine the vertical location.

Image
From: https://continuouswave.com/whaler/refer ... ingle.html

The Anti-Ventilation plate is located 2.5-inches HIGHER than the keel of the hull in that view. Later I raised the engine 0.75-inches higher, and the current set-up is with the A-V plate 3.25-inches above the keel of the hull. While this dimension may not be exactly proper for a RAGE 15 hull, it does give you the sense that the A-V plate should not be BELOW the keel.

Note also that the bottom of the Whaler Drive is sloping upward. It is not a flat surface, like a running pad.

dtmackey
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Re: Modified RAGE 15 to Outboard

Postby dtmackey » Tue Jul 13, 2021 9:15 am

Jim beat me to this, but the engine is buried in the water and you will not achieve a decent ride with the current setup. Porpoising is a common outcome for boats dragging too much lower unit in the water.

Generally speaking, shorter boats don't respond as well to brackets due to the weight of the motor being moved beyond the stern causing the stern to "hog" down in the water. Manufacturers typcially offer brackets on boats of 20-feet or longer, and they also account for this setback by raising the bracket on the transom so the motor sits higher.

The only options I see available are:

—switch to a 15-inch-shaft motor.

—raise the stern bracket on the transom to achieve the desired amount of lower unit in the water while on plane.

While on plane you should be able to see the top of the anti-ventilation plate The downside to raising the bracket is the wetted area of the buoyancy box is reduced and the boat will sit lower in the water due to weight being carried further off the stern.

The weight of that Mercury FOURSTROKE is already heavy for that boat at 360-lbs dry, so rigged and set you are approaching 480-lbs. I can't image that much weight on a bracketed boat that short.

Another option would be a lighter engine , a two-stroke Yamaha is only 225-lbs and the F70 Yamana is 255-lbs. Removing 100-lbs hanging off the boat that far back would make a difference in additional to options above.

The general idea on measuring the anti-ventilation plate to boat keel is an additional 1-inch of lift above the keel for every 12-inches of setback, but I've seen boats that respond to additional lift above this.

D-

jimh
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Re: Modified RAGE 15 to Outboard

Postby jimh » Tue Jul 13, 2021 3:23 pm

dtmackey wrote:The general idea on measuring the anti-ventilation plate to boat keel is an additional 1-inch of lift above the keel for every 12-inches of setback...


That formula seems appropriate for the Whaler Drive installation I show above. The engine setback distance is about 26-inches and the transom height is raised about 2.25-inches:

26/2.25= 11.55-to-1 ratio

which is just as you suggest is the normal layout.

jimh
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Re: Modified RAGE 15 to Outboard

Postby jimh » Tue Jul 13, 2021 3:29 pm

dtmackey wrote:...The weight of that Mercury FOURSTROKE is already heavy for that boat at 360-lbs dry, so rigged and set you are approaching 480-lbs. I can't image that much weight on a bracketed boat that short...

---Another option would be a lighter engine , a two-stroke Yamaha is only 225-lbs ...


I believe that back in the day when people were making RAGE jet drive boat to RAGE outboard engine boat conversions, most of the outboard engines used were lighter two-stroke-power-cycle engines. Also, the conversion would typically use a 10-inch setback bracket, not an entire stern bracket with swim platform and buoyancy box.

To learn the weight of the stern platform added to this RAGE 15 would be useful. Its buoyancy box may be trying to overcome much more weight than expected.

dtmackey
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Re: Modified RAGE 15 to Outboard

Postby dtmackey » Tue Jul 13, 2021 5:17 pm

jimh wrote:I believe that back in the day when people were making RAGE jet drive boat to RAGE outboard engine boat conversions, most of the outboard engines used were lighter two-stroke-power-cycle engines. Also, the conversion would typically use a 10-inch setback bracket, not an entire stern bracket with swim platform and buoyancy box.

To learn the weight of the stern platform added to this RAGE 15 would be useful. Its buoyancy box may be trying to overcome much more weight than expected.


The buoyancy box, or tub as referred to by bracket builders would only provide buoyance at rest and headway speed. As soon as the boat start to climb onto a plane the tub is typically not in contact with the water cause the motor to act as a cantilever on the boat pulling the transom down. If the boat is not set up properly, this can have an added impact the how the boat handles and rides.

I've seen many bracket installs on boats that were not oringlally factory outfitted and transom weight was the biggest contributor to how it rode. To that point, one friend traded in the newer twin motors Zuke 140hp motors he outfitted to the bracket and went with a single 2 stroke, saving 250# and his ride instantly improved on a 22' boat and that was with a twin tub (buoyancy box designed for twin motors)

D-

jimh
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Re: Modified RAGE 15 to Outboard

Postby jimh » Tue Jul 13, 2021 6:41 pm

Re: buoyancy box: I use that term because Bob Dougherty used that term when he explained to me his design that became the “Whaler Drive.”

Bob also said that it was called a “Dougherty Drive” around the factory while in development, but Marketing guys insisted on naming it the “Whaler Drive.”

Some twin-engine stern brackets do not have any buoyancy box; an example is the Armstrong bracket used on WHALE LURE. The twin V6 small-displacement Mercury EFI engines are relative light-weights by today’s standards.

WHALE LURE does have a decidedly bow-up static trim, and like all boats you can—if you try hard—get the bow to oscillate a bit at planing speeds.