Two Streams, Two Questions

Hi Randy, 

Thanks for writing tech talk.  My boats are away for the winter so its time for planning and dreaming. 

VOO DOO w/70 NISSAN redux. 

I wrote originally asking about propellers.  I thought that I would need more pitch than I could find.  This may be true. 

Original set up: 

no foot throttle
engine flat on transom
18" Merc chopper
50 mph gps at 5800
It was a scary ride for a newcomer to V bottoms.

New set up: 

Motor raised to the highest bolt position
Foot thottle installed
The top of the bullet is at about the bottom of the pad.
18" chopper
20" over hub 'dumbo'
Much more stable, but no faster

5800 rpm max with all conditions
Solo w/18" chopper = 46 mph
Two up with 20" = 48 mph
Solo with 20" =  51 mph

The boat runs fastest with neutral trim. 

The dumbo prop is an old Merc race prop from the early 70's.  The ears are pretty big.  It looks more like my Hoss Hyperdrive than a chopper. 

As far as set up goes, I think I'm done.  Is there anything else to try?   

If I ever find a used 60 Merc w/1.64 gears would it push the boat faster?


8" set back
solid mounts
G force tuner
Boyeson reeds

Bob's nose cone
24" Laser = 68 mph gps (solo)
26" Hoss = 72 mph solo (solo)

both at around 6000 rpm
passengers slow it down a lot, especially the Laser

The boat is mostly a cottage ski boat, but I do like to go fast on occasion.  Sometimes we take trips and have to buy 'lake' gas. 

The Laser is too much pitch for pulling my 200 lb brothers out on slalom skis.  At 175 lb, I pull out OK. 

Q1  Would a stock 200 Merc be worth the upgrade?  If I upgrade I would have two goals.  First, to be able to pull my brothers out and still see 70 mph with the same prop.  Second, to see close to 80 with a 'speed' prop.  I think the rev limiter on the new 200 carbs would limit the flexibility. 

Q2  How far down in pitch do I need to go to pull out my brother?  If I keep the XR4 I think I'll buy a Trophy. 

Q3  Would water ski duty (lots of idling and starting) constantly flatten the battery on a 2.5L? 

My local engine builder recommended not to 'hot rod' the XR4. 

Thanks for any suggestions 

David Funnell
Windsor, Ontario



Voodoo stats: for some reason, I'd look at the pad and hull for soft spots and hooks. Your speed seems about 10 mph off the pace. Should be in the low 60's. The hint is that the 18" chopper goes slower after you raised it up. Also the 70 Nissan has an electronic rev limiter at, guess what, 5800. I'll research for you, but if I'm not mistaken, they have a disconnect wire like Yamaha uses and we just need to find what wire it is. The Dumbo prop is an old Merc "offshore" prop. Good prop for engines, find an old 75 hp short shaft Johnson or Evinrude "E" class motor. They are less than $1000 used around here and with the 18" chopper, we had one running 68 on a Vixen at 7400 rpm. As for the Virage, the best thing you could do is get the larger 1:86 ratio gearcase. The little XR4 1:78 gearcase doesn't carry the boat very well. Next just port the XR4 to 200 spec and you'll have what you desire. You can re-jet the carbs or find a set off a late model 2.4 200 and just bolt them on. Both of your props will be perfect for skiing and the speed runs. But this is with the big gearcase. Sounds like the local builder doesn't quite know how to 200ize the XR4. This would explain his reluctance, as it is a simple operation. If you go the "new" 200 route, the rev limiter is a control box on the powerhead and can be removed. The 2.5 has the 40 amp charge system like your XR4, so no difference. 




Please submit whatever questions you have to  All questions and answers selected will be displayed on this page as they come in.  Note: though all boaters are welcome, priority may be given to I.H.R. members.  Randy can not be held responsible for any advice given.  Though his information and expertise is second to none, he has no control over what you do with your boat.  It is up to you to boat safely and act responsibly, and his advice is only to be used as a guidance for your high performance boat/motor of which you are the one responsible for the risks involved.

Mark C.


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