How Much To Go Faster

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This craft was built in around 1974, one of around 60 Mk I Corsairs built by Salthouse’s in the halcyon days prior to the Muldoon boat tax, which essentially scuppered the NZ boat building industry in a short-term tax grab on the discretionary dollar.

Engine options in those days included the choice of either a 3160 or 3208 Caterpillar V8, or the 6V53 or 6-71 Detroit Diesel (the Gimmy), nominal 205-210hp powerplants. The 6-71 produced 275hp but came with the caveat of reduced headroom, and a sloping floor in order to fit underfloor without an engine box. Today’s modern diesels have a very much higher power-to-weight ratio than these two old behemoths, but they are/were normally very reliable.

The Corsair had a (then) light construction laminate of polyester FRP resin, fibreglass gunstock, hand laid mat and a balsa core and had a displacement of around 8.5 tonnes. They would touch 20 knots when first they touched the water, and could still manage 18 knots two weeks later when fuel, water, supplies, refreshments and fishing tackle were introduced, and consequently developed a reliable 13-14 knot top end cruise in normal trim around Christmas time. Running a 1.5:1 or 2:1 gearbox, on a straight shaft drive, with a full keel and rudder shoe for protection, the Mk I Corsair was the benchmark for all midsized planing craft in that era, and still enjoys an enviable reputation today.

BUT, there were problems on the horizon… Polyester gunstock laminates developed an alarming tendency to ‘wick’, swallowing copious quantities of water resulting in the dreaded marine acne, or ‘The Pox’ – more properly known as osmosis. Combine that with balsa’s amazing ability to absorb moisture that crept in via skin fittings, keel bolts and anywhere a perforation had been made in the core, and soon these boats were carrying an extra few hundredweight in the area between outer and inner laminates. (For those who have either forgotten, or were born after 1980, a hundredweight is 1/20 of an imperial ton, or approximately 51kg.) Game fishing poles, game chair and the huge amount of paraphernalia needed to survive on the water whilst fishing added even more weight. Live bait tanks were often added, putting an extra 150kg right at the back of the boat when full.

But that’s OK, waterline trim was equalised by the change from rope/chain winches on the front, to full chain auto anchor units adding another few kg way in the front of the boat. A small paint modification to the boot top & waterline levels covered up that sin. The beautifully quiet, soft chine, designed by Bob Salthouse, disappeared below the water. By this stage, our once proud wee craft is now a portly 10-tonne-plus porker. Honest cruise speed was now around 11 knots, which meant that in real terms, she was just on the plane and not doing it well, or with any meaningful reserves of performance. It is well documented that the Corsair hull form is unparalleled in any sea state forward of the beam, (albeit a bit wet), good in a beam sea, and an absolute handful in anything aft of the beam. The combination of her fine bow, maximum waterline beam well aft, full keel, smallish rudder and (by this stage), inadequate horsepower and her looming obesity meant that any well-meaning stern quarter wave would cause her to wallow initially, then flick into a bow down trim as the wave passed under her. Depending on where the bow was pointing at that stage you either went to port, starboard or in some cases, back the way you came from, all without any meaningful input from the helmsperson. A straight stern wave of any size was the ultimate adrenalin rush. The stern would lift, the bow would go down, and Mr Businessman was off!! If driving from the downstairs helm, with the helm window open, it was not uncommon to dip one’s elbow into the water as your beautiful craft ignored full opposite lock, and what was left of the horsepower in your motor, and turned across the face of the wave in whichever direction it chose, on an angle of heel many sailboats would be uncomfortable with. Being on the flybridge multiplied this effect in quantum leaps.

At some stage, this Mk I Corsair had annoyed, frustrated or frightened her owner sufficiently that the first tentative steps were taken to resolve the minor character flaws in what was otherwise an excellent cruising craft. He is told that making her lighter will help. To that end she is removed from the water, placed on the hardstand, has holes drilled in her stern to drain any waters that may be trapped in the balsa, and has her outer layers of skin removed, along with any osmotic pimples. Leaving her to dry out naturally was the norm then. It was the early days of osmotic treatment, and many were the methods. Each resin or paint supplier had the cure, some nearly worked, but all consisted of significant labour, hardstand and material costs. A $16,000 bill saw our craft back in the water, pox free, somewhat lighter, and cruising back at 14kts… for a while!

‘Reduce drag for more performance’ is the next catch phrase, and get rid of the aft quarter helming issues at the same time. To that end, along with many of the Mk I’s, our next owner, for a small $8k figure, removes the shoe under the rudder, and cuts some of the keel off. Many have taken most, if not all, of the keel off, which if combined with a more efficient rudder form did assist somewhat in downwind handling. Of course, without that keel the boats then swung a lot more at anchor, and hated lying to the current when fishing the tide. All the while, boat speed has increased by @ 1kt when in normal trim, so our noble craft is now doing 12-13kts cruise again… for a while!

“Ah ha” cries an expert, as our owner bewails about modern cruising catamarans sailing past him. The Corsair hull form squats at around a 7 degree angle when on the plane, dragging water when cruising at 11kts. Compounded by all her extra weight, this forces the use of excessive trim tab to level her out – more drag!” Let’s extend the waterline length of her, by building in under the boarding platform. This will alleviate the squatting issue and give more speed for the same horsepower”, spake the expert. Our then Mk I owner hears these sage words and for a miserly $8000 has the boarding platform and underneath extended by a metre. This has another bonus in that the bait tank is now huge, carrying much more water (weight) when full. Our owner puts his modified and modernised craft back in the water and sure enough, his speed has increased by 1.5 knots, but there is a catch. Bob Salthouse deliberately designed this hull to run at 7 degrees. This put much of the narrow bow and deep forefoot out of the water when on the plane, giving a flatter surface to plane on, and better steerage.


Our new 1m extension to her underwater profile, and the lift the extra buoyancy creates has dropped her angle of plane down to 3 degrees or even less. In ideal conditions she is quicker, but now that hull form’s tendency to steer by the bow is exacerbated in both upwind and downwind angles, and she is as wet as a shag. She does not lift to a wave anymore, and the spray pattern is impressive. Sea water is all over the boat, all the time. No problem, our owner states, and for a quick $4k he gets rid of the useless sun dodger and installs a full headroom, fully enclosed bimini. Although now mainly dry, he bemoans the fact that the extra windage has taken back at least 50% of his speed gains, and she is not so easy to berth in a breeze any more. A modern bowthruster is only around $10k installed, so that problem is solved in short order….

Over the next few seasons and some new owners, she steadily slows down to the 11-knot cruise figure again. It is a well-known fact that nothing ever comes off a boat, it only goes on, like weight around the hips and belly. Her wee 6V53 is now 25 years old, with around 5000 hours on the clock. It is probably still good for 180hp in a short burst but is much happier at 9-10 knots in a semi displacement mode than sprinting. It is weeping from many and various orifae too. A quick $15k sees the Gimmy restored to 205hp, but while she was out on the hard for those weeks, the then owner noted that some parts of the underwater areas were never dry, in fact bleeding around keel bolts and skin fittings. A biopsy with the new-fangled moisture meter reveals that the laminate and balsa core are saturated again, and the lifetime osmotic repair has died. A season and $25,000 later, our re-hulled, repainted pristine Mk I hits the tide, and hits flank speed of 16kts on trials. Our disgruntled and disillusioned owner puts her on the market. Various owners ensue over the next few years.


For the thick end of his first season he happily stooges around the Hauraki Gulf, the little Detroit singing its distinctive song, but he tires of being late to the best anchorages. This man is of a practical engineering background, is well versed in things nautical, and surmises there must be more performance to be gained from this craft. He only wants a knot or two….!

He starts with the engine. Is it giving its entire horsepower? He has well respected experts do running tests, and discovers the exhausts are creating back pressure. Custom crafted S/S exhaust risers for $4000 do wonderful things, the engine runs 5 degrees cooler, the extra water thru the system takes away some of the Detroit drone, and performance is exactly the same.


Now this man is an ex yachtie. He knows how to feel the boat, anticipate a wave and apply judicious amounts of lock in a timely manner to keep her on a relatively straight path. The increase in loadings on the wee rudder causes a bolt on the rudder shaft to shear while
running downhill in a 25-knot SW breeze off Owhanake. A very character-forming moment ensued as his craft directed herself across the face of the waves, directly at some nearby and very hard pieces of Waiheke.

Crisis averted by way of a friendly tow, our much chastened owner soldiers on through the season, developing powerful forearms and shoulders. The issue reaches its zenith when a 30-knot NW wind up the blurter makes the wide Guv’nors Passage at Great Barrier almost too narrow for his miscreant craft. A decision is made to remove much of the extended waterline under the transom. While on the hard, the discovery is made that the removal of the rudder shoe many years ago was done without the lengthening of the rudder stock and the addition of a top bearing to support the system. This has resulted in the current supports leaving their location when under load and allowing the rudder to cant over like an airplane wing, giving lift an America’s Cup designer dreams of. No wonder she can’t be steered downhill, he muses, and a quick $6000 in boatbuilding, engineering and paint repair sees his shortened craft back in her berth. Her handling is much improved, the bow actually lifts to a wave instead of bulldozing it and spray is reduced to tolerable levels. BUT… performance is essentially the same.

More research, multiple readings of the late Len Gilbert’s Diesel Dairy, and employing John Menzies to strap on his gadgets, take him to where he and many before him should have gone first-up.

John’s expertise proves the 6V53 is doing exactly what it should, using exactly the amount of fuel it should, and delivering the exact horsepower requested by the throttle man. It is when it gets to the wheel (propeller) that the main issue becomes apparent.


Just as it is stupid to drive on a flat tyre, so too is it useless to expect much from a bad prop. The old 24 x 20 3-blade propeller fitted has seen logs, sandy and muddy bottoms, and has been de zinced a bit when anodes were missed. It is very tired. So much so that it has 38% slip. If there were no slip, each revolution of the prop would move the boat forward 20”. The old wheel is moving it 12.4” forward, and that is on a good day

Consultations with a well-known North Shore propeller supplier, and the payment of $6000 saw a modern 4-blade Tiger prop measuring 23” diameter x 19” pitch fitted. The change is electric. Our noble Mk I, running at @ 7 degrees of angle is now doing 16.5kts flat out and cruising at 15kts at the top of cruise revs, and maximising the improvements with a few revs to be gained by a small de-pitch and losing a bit of diameter. This prop mod does give that final incremental increase in speed to 16.9kt, but cruise speed is down at the same revs, to 14.5kts. At $1000 a time for travelift fees, and re-pitching our disgruntled owner takes the decision that that will do for now.

After the arrival of her new prop, the valiant 6V53, with about 2000 hours on her since the rebuild, happily sings her two-stroke song for about…… 1 hour. The efficiency and consequent load requirement of the new prop proves too much for the now 39-year-old motor, and she throws a leg out of bed in protest. The resultant knocking is drowned out by our owners heartbeat at this stage, as various experts diagnose there is no ‘cheap fix’ for this problem. Despite the ability of the Detroit mechanic to insert 1 new piston, liner and assorted bits to remedy the issue, all say that the other 5 cylinders should be done too. Rebuild cost estimates vary from $21,000 to $25,000 in today’s modern economic climate. At the end of this, our owner will have a brand new 40-year-old motor, doing exactly what it should, and giving performance exactly where it left off. Much wailing and gnashing of teeth later, and after exhausting all the easy options like second hand transplants, our owner decides to bite the bullet. A factory rebuilt 325hp Cummins 6BT is purchased, sans gearbox, for $36,000. Further consultations with the propeller manufacturer shows that the brand new prop can be modified to handle this extra horsepower, with a 1.8:1 gearbox fitted. This gearbox is duly acquired for $12,500 fitted to the Cummins.

Being a straight-6 motor in contrast to the V6, modifications to the exhaust system are required. This includes a blow-by relief skin fitting for the extra volumes of air and water being pushed down it, rather than running an all-new exhaust. Our owner’s wallet is cringing by this stage. Luckily, the Corsairs were over-built and the shaft is of a size that will handle the extra 120hp being delivered to it. Because nothing is easy in things nautical, the down-angle gearbox fitted to the new motor means modifications to the engine beds to give around 1degree of fall to the front end of the motor. This is well within tolerance of 5deg allowed by Cummins, so the shaft angle does not need to be re drilled. A battle with wiring looms, throttle and gearbox cabling, freezer compressor mountings and a myriad of small details like a new dash for the instruments means the cost to date is now $65,000, with the bill from the engine installers to come. This was estimated at around $6,000 and to take a week. Some six weeks later, our proud but battered owner, having fitted his re machined and re pitched 22.5 x 21.5 prop, re launches his pride and joy to avoid further hardstand costs. Final connections take place in her berth. D Day arrives!! The motor is quiet and smooth, the exhaust back pressure is OK and the boat does not go into reverse when driven out of its berth for first trials. Thank heavens for that now essential cruising aid, the bow thruster. Crisis narrowly averted again!

This revitalised vessel, with a motor half the size and weight of the old unit, a prop twice as efficient as that previously employed, and another 120hp to play with, now scorches along at 21.5kts max and cruises at 16.5kts in half load trim. The wide flat stern wave as seen in his pictures is only narrow in comparison to the ear to ear grin on his visage. Wifely glowering and bank managers’ plaintive calls are irrelevant to the joy of passing that ‘other B……’


BUT… there are dark clouds on the horizon. It seems ‘if we remove the rest of the keel, we might pick up a knot or two..!’

I have another friend who also has an old Mk1 Corsair. This beauty is original in every way, right down to her old 6V53 Detroit. He cruises everywhere at 9-11 knots, and his only real expense other than routine preventative maintenance over the past 10 years has been food and grog. An ex yachtie too, he figures 10 knots VMG is good, and if it’s blowing from the wrong direction, he doesn’t go. Problem solved!!


Have a question? Ask away..

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